Supplement Forms/Alternate Names
• Ascorbate; Ascorbic Acid
Principal Proposed Uses
• None
Other Proposed Uses
• Acute Anterior Uveitis; Aging Skin; Allergies; Asthma; Autism; Bedsores; Bladder Infections in Pregnancy; Cancer (Prevention); Cancer (Treatment); Cataracts; Colds (Treatment, Not Prevention); Easy Bruising; Gallbladder Disease (Prevention); Glaucoma; Heart Disease (Prevention); HIV Support; Hypertension; Insomnia; Low Sperm Count; Macular Degeneration; Maintaining Effectiveness of Nitrate Drugs; Menopausal Symptoms; Minor Injuries; Muscle Soreness after Exercise; Osteoarthritis; Photosensitivity; Pneumonia; Preeclampsia (Prevention); Reflex Sympathetic Dystrophy (RSD) (Prevention); Sunburn (Prevention); Vascular Dementia (Prevention); Ulcers; Weight Loss
Probably Not Effective Uses
• Cervical Dysplasia; Common Cold (Prevention)
Although most animals can make vitamin C from scratch, humans have lost the ability over the course of evolution. We must get it from food, chiefly fresh fruits and vegetables. One of this vitamin's main functions is helping the body manufacture collagen, a key protein in our connective tissues, cartilage, and tendons.
From ancient times through the early nineteenth century, sailors and others deprived of fresh fruits and vegetables developed a disease called scurvy. Scurvy involves so-called scorbutic symptoms, which include nonhealing wounds, bleeding gums, bruising, and overall weakness. Now we know that scurvy is nothing more than vitamin C deficiency.
Scurvy was successfully treated with citrus fruit during the mid-1700s. In 1928, when Albert Szent-Gyorgyi isolated the active ingredient, he called it the anti-scorbutic principle, or ascorbic acid. This, of course, is vitamin C.
Vitamin C is a powerful antioxidant that neutralizes damaging natural substances called free radicals. It works in water, both inside and outside of cells. Vitamin C complements another antioxidant vitamin, vitamin E, which works in lipid (fatty) parts of the body.
Vitamin C is the single most popular vitamin supplement in the United States and perhaps the most controversial, as well. In the 1960s, two-time Nobel Prize winner Dr. Linus Pauling claimed that vitamin C could effectively treat both cancer and the common cold. Subsequent research has mostly discounted these claims, but hasn't dampened enthusiasm for this essential nutrient. The vitamin C movement has led to hundreds of clinical studies testing the vitamin on dozens of illnesses; at present, however, no dramatic benefits have been discerned.
Scurvy, the classic vitamin C deficiency disease, is now a rarity in the developed world, although a more subtle deficiency of vitamin C is fairly common.2-6 According to one study, 40% of Americans do not get enough vitamin C.7 In fact, vitamin C deficiency sufficient to cause bleeding problems during surgery turns out to be more common than previously thought.141
Aspirin and other anti-inflammatory drugs might lower body levels of vitamin C,8-10 as might oral contraceptives.11-15 Supplementation may be helpful if you are taking any of these medications.
Most of us think of orange juice as the quintessential source of vitamin C, but many vegetables are actually even richer sources. Red chili peppers, sweet peppers, kale, parsley, collard, and turnip greens are full of vitamin C, as are broccoli, Brussel sprouts, watercress, cauliflower, cabbage, and strawberries. (Oranges and other citrus fruits are good sources, too.)
One great advantage of getting vitamin C from foods rather than from supplements is that you will get many other potentially healthful nutrients at the same time, such as bioflavonoids and carotenes. However, vitamin C in food is partially destroyed by cooking and exposure to air, so for maximum nutritional benefit you might want to try freshly made salads rather than dishes that require a lot of cooking.
Vitamin C supplements are available in two forms: ascorbic acid and ascorbate. The latter is less intensely sour.
However, there are some flaws in this research. It is possible that vitamin C levels might rise in other tissues even if they remain constant in the blood. Furthermore, this study did not evaluate the possible effects of taking vitamin C several times daily rather than once daily.
Many nutritional experts recommend a total of 500 mg of vitamin C daily. This dose is almost undoubtedly safe. Others recommend that you take as much vitamin C as you can, up to 30,000 mg daily, cutting back only when you start to develop stomach cramps and diarrhea. This recommendation seems based more on a semi-religious enthusiasm for the vitamin C than on any evidence that such huge doses of the vitamin are good for you.
Intravenous vitamin C can easily raise vitamin C levels to a level 140 times higher than the maximum achievable with oral vitamin C.152 However, there is no meaningful evidence that intravenous vitamin C provides any medical benefits.
Regular use of vitamin C does not seem to help prevent colds.86,146,185 One exception is the “post-marathon sniffle”—colds that develop after heavy exercise.19,20 Vitamin C may be helpful for preventing this condition, although not all studies agree.153
A review of 3 randomized studies involving 2,335 people found that vitamin C may reduce the risk of pneumonia in adolescents and adults, .187 The benefits of vitamin C appeared to be largely restricted to people at an increased risk of getting pneumonia and those with low dietary intake of the vitamin. The authors concluded that the widespread use of supplemental vitamin C to prevent pneumonia was not supported by their review.
Two double-blind studies suggest that the use of vitamin C combined with vitamin E might slightly reduce the risk of developing preeclampsia, a complication of pregnancy.21,169 However, a much larger follow-up study failed to find benefits.170
Two studies conducted by a single research group have found that vitamin C at a dose of 500 mg daily might help prevent reflex sympathetic dystrophy, a poorly understood condition that can follow injuries such as fractures.110,180
Over time, the body develops tolerance to drugs in the nitrate family (such as nitroglycerin). Some evidence suggests that use of vitamin C can help maintain the effectiveness of these medications.142-145
Other small double-blind trials suggest that vitamin C might be helpful for anterior uveitis (when taken in combination with vitamin E),52 autism,40 easy bruising,29 minor injuries,41 protecting the liver in nonalcoholic steatohepatitis,154 speeding recovery from bedsores,37 treating female infertility (specifically, a condition called "luteal phase defect"),155 and preventing early rupture of the chorioamniotic membranes ("the water breaking") in pregnancy.156 Vitamin C might also improve the effectiveness of antibiotic treatment for Helicobacter pylori, the cause of most peptic ulcers.179 Given the limited nature of this research, however, further studies are needed to conclude that vitamin C is beneficial for these conditions.
Preliminary evidence suggests that cream containing vitamin C may improve the appearance of aging or sun-damaged skin.50,51,157 Inconsistent evidence suggests that oral or topical vitamin C, taken by itself or in combination with vitamin E, may also help protect the skin against sun damage.42-49
Double-blind studies of vitamin C for the following conditions have yielded mixed results: asthma,30, 178 male infertility,38,39 reducing the muscle soreness that typically develops after exercise,36,176,177 and hypertension.31-35,147,148 Note: Unexpectedly, one study found that a combination of vitamin C (500 mg daily) and grape seed oligomeric proanthocyanidins (OPCs) (1,000 mg daily) slightly increased blood pressure.158 Whether this was a fluke of statistics or a real combined effect remains unclear.
Limited and in some cases contradictory evidence suggests possible benefit in the prevention or treatment the following conditions:
Observational studies indicate that people with a higher intake of vitamin C have a lower incidence of macular degeneration, heart disease, cancer, and osteoarthritis.22-27,59-66 However, these findings do not indicate that vitamin C supplements will help prevent or treat these conditions. Observational studies are notoriously unreliable for showing the efficacy of treatments; only double-blind studies can do that. (For more information on why double-blind studies are so important, see Why Does This Database Rely on Double-blind Studies? ) Two large double-blind trials exploring the effectiveness of vitamin C for heart disease prevention—one in women at high risk 181 and the other in men at low risk— 183 failed to find any benefit at all.
Vitamin C has been proposed as a treatment for cancer, but this claim is very controversial, and there is as yet no scientifically meaningful evidence that it works.67-70
Massive doses of vitamin C have at times been popular among people with HIV infection based on highly preliminary evidence.71,72 An observational study linked high doses of vitamin C with slower progression to AIDS.73 However, a double-blind study of 49 people with HIV who took combined vitamins C and E or placebo for 3 months did not show any significant effects on the amount of HIV detected or the number of opportunistic infections.74 Furthermore, one study found that vitamin C at a dose of 1 g daily substantially reduced blood levels of the drug indinavir, a protease inhibitor used for the treatment of HIV infection.160 This could potentially cause the drug to fail.
A large, 8-year trial involving 11,545 physicians aged 50 years and older concluded that vitamins C and E, alone or in combination, did not lower the risk of cataracts.186
In a study of 80 women with Chlamydia trachomatis infection, adding vitamin C to doxycycline and triple sulfa vaginal cream reduced discharge and pain associated with intercourse.184
One substantial study failed to find vitamin C useful for improving high cholesterol.161
According to a double-blind, placebo-controlled study of 141 women with cervical dysplasia (early cervical cancer), vitamin C, taken at a dosage of 500 mg daily, does not help to reverse the dysplasia.75
Vitamin C also does not appear to be helpful for treating Raynaud’s phenomenon caused by scleroderma.150
However, a cool-headed evaluation of the research indicates something in between. Numerous studies have found that vitamin C supplements taken at a dose of 1,000 mg daily or more throughout the cold season can modestly reduce symptoms of colds and help you get over a cold faster, but they do not generally help prevent colds.81,83,146,162,185
Many people use vitamin C for colds in a different way: they only begin taking it when cold symptoms start. Vitamin C is probably not effective when used in this way.83,140,185 One double-blind trial enrolled 400 individuals with new-onset cold symptoms, and divided them into four different daily vitamin C dosage groups: 30 mg daily (a dose lower than the minimum daily requirement, and used by the researchers as a placebo), 1,000 mg, 3,000 mg, or 3,000 mg with bioflavonoids.140 Participants were instructed to take the vitamin at the onset of symptoms and for the following 2 days. The results showed no difference in the duration or severity of cold symptoms among the groups. High-dose vitamin C taken at the onset of a cold, in other words, didn't help. A review of 7 randomized and non-randomized trials also found that taking vitamin C at the start of a cold did not offer any benefits.185 The bottom line: If you want to use vitamin C to take the edge off your colds, take the supplement throughout the winter.
There are numerous other natural treatments for the common cold as well, some of which may be more helpful than vitamin C. For more information, see the full Colds and Flus article.
However, people who are truly vitamin C-deficient, such as elderly people in nursing homes, may show increased resistance to infection if they take vitamin C (or other nutrients).86
In addition, vitamin C might be helpful for preventing the respiratory infections that can follow heavy endurance exercise. Marathon running and similar forms of exertion can temporarily weaken the immune system, leading to infections. Vitamin C may be helpful. According to a double-blind, placebo-controlled study involving 92 runners, taking 600 mg of vitamin C for 21 days prior to a race, made a significant difference in the incidence of sickness afterwards.87 Within 2 weeks of the race, 68% of the runners taking placebo developed cold symptoms versus only 33% of those taking the vitamin C supplement. As part of the same study, non-runners of similar age and gender to those running were also given vitamin C or placebo. Interestingly, the supplement had no apparent effect on the incidence of upper respiratory infections in this group. Vitamin C seemed to be effective in this capacity only for those who exercised intensively!
Two other studies found that vitamin C could reduce the number of colds experienced by groups of people involved in rigorous exercise in extremely cold environments.88 One study involved 139 children attending a skiing camp in the Swiss Alps, while the other enrolled 56 military men engaged in a training exercise in northern Canada during the winter months. In both cases, the participants took either 1 g of vitamin C or placebo daily at the time their training program began. Cold symptoms were monitored for 1 to 2 weeks following training, and significant differences in favor of vitamin C were found.
However, one very large study of 674 US Marine recruits in basic training found no such benefit.89 The results showed no difference in the number of colds between the treatment and placebo groups.
What's the explanation for this discrepancy? There are many possibilities. Perhaps basic training in the Marines is significantly different from the other forms of exercise studied. Another point to consider is that the Marines didn't start taking vitamin C right at the beginning of training, but waited 3 weeks. The study also lasted a bit longer than the positive studies mentioned above, continuing for 2 months; maybe vitamin C is more effective at preventing colds in the short term. Of course, another possibility is that it doesn't really work. More research is needed to know for sure.
While this research is promising, larger studies are necessary to confirm whether vitamins C and E will actually work. The authors of this study point out that similarly sized studies found benefits with other treatments, such as aspirin, that later proved to be ineffective when large-scale studies were performed. Furthermore, keep in mind that we don't know whether such high dosages of these vitamins are absolutely safe for pregnant women.
Two studies performed by a single research group reported evidence that vitamin C can help prevent RSD after wrist fractures.110,180
In one of these, a total of 123 adults with wrist fractures were enrolled and followed for one year.110 All were given 500 mg of vitamin C or placebo daily for 50 days. The results showed significantly fewer cases of RSD in the treated group.
A subsequent study conducted by the same research group compared placebo against three dosages of vitamin C in 416 people suffering from wrist fracture.180 Again, treatment continued for 50 days. The results indicated that approximately 10% of those given placebo developed RSD, while less than 2% of those given either 500 mg or 1,500 mg of vitamin C daily did so. According to the statistical analysis used by the authors, this relative benefit was statistically significant. The 200 mg dose of vitamin C did appear to offer some protection too, but not as much.
Note: Angina is too serious a disease for self-treatment. If you have angina, do not take vitamin C (or any other supplement) except on a physician’s advice.
Concerns have been raised that long-term vitamin C treatment can cause kidney stones.129,130,163 However, in large-scale observational studies, individuals who consume large amounts of vitamin C have shown either no change or a decreased risk of kidney stone formation.131-133 Still, there may be certain individuals who are particularly at risk for vitamin C-induced kidney stones.134 People with a history of kidney stones and those with kidney failure who have a defect in vitamin C or oxalate metabolism should probably restrict vitamin C intake to approximately 100 mg daily. You should also avoid high-dose vitamin C if you have glucose-6-phosphate dehydrogenase deficiency, iron overload, or a history of intestinal surgery.
Vitamin C supplements increase absorption of iron.121-128,164 Since it isn’t good to get more iron than you need, individuals using iron supplements shouldn’t take vitamin C at the same time except under a physician’s supervision.
One study from the 1970s suggests that very high doses of vitamin C (3 g daily) might increase the levels of acetaminophen (such as Tylenol) in the body.135 This could potentially put you at higher risk for acetaminophen toxicity. This interaction is probably relatively unimportant when acetaminophen is taken in single doses for pain and fever, or for a few days during a cold. However, if you use acetaminophen daily or have kidney or liver problems, simultaneous use of high-dose vitamin C is probably not advisable.
Weak evidence suggests that vitamin C, when taken in high doses, might reduce the blood-thinning effects of warfarin (Coumadin) and heparin.136-139
As noted above, one study found that vitamin C at a dose of 1 g daily substantially reduced blood levels of the drug indinavir, a protease inhibitor used for the treatment of HIV infection.165
Heated disagreement exists regarding whether it is safe or appropriate to combine antioxidants such as vitamin C with standard chemotherapy drugs. The reasoning behind the concern is that some chemotherapy drugs may work in part by creating free radicals that destroy cancer cells, and antioxidants might interfere with this beneficial effect.76,166 However, there is no good evidence that antioxidants actually interfere with chemotherapy drugs, but there is growing evidence that they do not.77,79,151
The maximum safe dosages of vitamin C for people with severe liver or kidney disease have not been determined.
• Ascorbate; Ascorbic Acid
Principal Proposed Uses
• None
Other Proposed Uses
• Acute Anterior Uveitis; Aging Skin; Allergies; Asthma; Autism; Bedsores; Bladder Infections in Pregnancy; Cancer (Prevention); Cancer (Treatment); Cataracts; Colds (Treatment, Not Prevention); Easy Bruising; Gallbladder Disease (Prevention); Glaucoma; Heart Disease (Prevention); HIV Support; Hypertension; Insomnia; Low Sperm Count; Macular Degeneration; Maintaining Effectiveness of Nitrate Drugs; Menopausal Symptoms; Minor Injuries; Muscle Soreness after Exercise; Osteoarthritis; Photosensitivity; Pneumonia; Preeclampsia (Prevention); Reflex Sympathetic Dystrophy (RSD) (Prevention); Sunburn (Prevention); Vascular Dementia (Prevention); Ulcers; Weight Loss
Probably Not Effective Uses
• Cervical Dysplasia; Common Cold (Prevention)
Although most animals can make vitamin C from scratch, humans have lost the ability over the course of evolution. We must get it from food, chiefly fresh fruits and vegetables. One of this vitamin's main functions is helping the body manufacture collagen, a key protein in our connective tissues, cartilage, and tendons.
From ancient times through the early nineteenth century, sailors and others deprived of fresh fruits and vegetables developed a disease called scurvy. Scurvy involves so-called scorbutic symptoms, which include nonhealing wounds, bleeding gums, bruising, and overall weakness. Now we know that scurvy is nothing more than vitamin C deficiency.
Scurvy was successfully treated with citrus fruit during the mid-1700s. In 1928, when Albert Szent-Gyorgyi isolated the active ingredient, he called it the anti-scorbutic principle, or ascorbic acid. This, of course, is vitamin C.
Vitamin C is a powerful antioxidant that neutralizes damaging natural substances called free radicals. It works in water, both inside and outside of cells. Vitamin C complements another antioxidant vitamin, vitamin E, which works in lipid (fatty) parts of the body.
Vitamin C is the single most popular vitamin supplement in the United States and perhaps the most controversial, as well. In the 1960s, two-time Nobel Prize winner Dr. Linus Pauling claimed that vitamin C could effectively treat both cancer and the common cold. Subsequent research has mostly discounted these claims, but hasn't dampened enthusiasm for this essential nutrient. The vitamin C movement has led to hundreds of clinical studies testing the vitamin on dozens of illnesses; at present, however, no dramatic benefits have been discerned.
Requirements/Sources
Vitamin C is an essential nutrient that must be obtained from food or supplements; the body cannot manufacture it. The official US and Canadian recommendations for daily intake are as follows:- Infants
- 0-6 months: 40 mg
- 7-12 months: 50 mg
- Children
- 1-3 years: 15 mg
- 4-8 years: 25 mg
- 9-13 years: 45 mg
- Males
- 14-18 years: 75 mg
- 19 years and older: 90 mg
- Females
- 14-18 years: 65 mg
- 19 years and older: 75 mg
- Pregnant Women
- 18 years old or younger: 80 mg
- 19 years and older: 85 mg
- Nursing Women
- 18 years old or younger: 115 mg
- 19 years and older: 120 mg
Scurvy, the classic vitamin C deficiency disease, is now a rarity in the developed world, although a more subtle deficiency of vitamin C is fairly common.2-6 According to one study, 40% of Americans do not get enough vitamin C.7 In fact, vitamin C deficiency sufficient to cause bleeding problems during surgery turns out to be more common than previously thought.141
Aspirin and other anti-inflammatory drugs might lower body levels of vitamin C,8-10 as might oral contraceptives.11-15 Supplementation may be helpful if you are taking any of these medications.
Most of us think of orange juice as the quintessential source of vitamin C, but many vegetables are actually even richer sources. Red chili peppers, sweet peppers, kale, parsley, collard, and turnip greens are full of vitamin C, as are broccoli, Brussel sprouts, watercress, cauliflower, cabbage, and strawberries. (Oranges and other citrus fruits are good sources, too.)
One great advantage of getting vitamin C from foods rather than from supplements is that you will get many other potentially healthful nutrients at the same time, such as bioflavonoids and carotenes. However, vitamin C in food is partially destroyed by cooking and exposure to air, so for maximum nutritional benefit you might want to try freshly made salads rather than dishes that require a lot of cooking.
Vitamin C supplements are available in two forms: ascorbic acid and ascorbate. The latter is less intensely sour.
Therapeutic Dosages
Ever since Linus Pauling, proponents have recommended taking vitamin C in enormous doses, as high as 20,000 mg to 30,000 mg daily. However, some evidence suggests that there might not be any reason to take more than 200 mg of vitamin C daily (10 to 100 times less than the amount recommended by vitamin C proponents).16 The reason is that if you consume more than 200 mg daily (researchers have tested up to 2,500 mg) your kidneys begin to excrete the excess at a steadily increasing rate, matching the increased dose. Your digestive tract also stops absorbing it well. The net effect is that no matter how much you take, your blood levels of vitamin C don't increase very much.However, there are some flaws in this research. It is possible that vitamin C levels might rise in other tissues even if they remain constant in the blood. Furthermore, this study did not evaluate the possible effects of taking vitamin C several times daily rather than once daily.
Many nutritional experts recommend a total of 500 mg of vitamin C daily. This dose is almost undoubtedly safe. Others recommend that you take as much vitamin C as you can, up to 30,000 mg daily, cutting back only when you start to develop stomach cramps and diarrhea. This recommendation seems based more on a semi-religious enthusiasm for the vitamin C than on any evidence that such huge doses of the vitamin are good for you.
Intravenous vitamin C can easily raise vitamin C levels to a level 140 times higher than the maximum achievable with oral vitamin C.152 However, there is no meaningful evidence that intravenous vitamin C provides any medical benefits.
Therapeutic Uses
According to numerous double-blind, placebo-controlled studies, regular use of vitamin C supplements can slightly reduce symptoms of colds and modestly shorten the length of the illness.17,18,146,185 However, taking vitamin C at the onset of a cold probably does not work.83,140, 185Regular use of vitamin C does not seem to help prevent colds.86,146,185 One exception is the “post-marathon sniffle”—colds that develop after heavy exercise.19,20 Vitamin C may be helpful for preventing this condition, although not all studies agree.153
A review of 3 randomized studies involving 2,335 people found that vitamin C may reduce the risk of pneumonia in adolescents and adults, .187 The benefits of vitamin C appeared to be largely restricted to people at an increased risk of getting pneumonia and those with low dietary intake of the vitamin. The authors concluded that the widespread use of supplemental vitamin C to prevent pneumonia was not supported by their review.
Two double-blind studies suggest that the use of vitamin C combined with vitamin E might slightly reduce the risk of developing preeclampsia, a complication of pregnancy.21,169 However, a much larger follow-up study failed to find benefits.170
Two studies conducted by a single research group have found that vitamin C at a dose of 500 mg daily might help prevent reflex sympathetic dystrophy, a poorly understood condition that can follow injuries such as fractures.110,180
Over time, the body develops tolerance to drugs in the nitrate family (such as nitroglycerin). Some evidence suggests that use of vitamin C can help maintain the effectiveness of these medications.142-145
Other small double-blind trials suggest that vitamin C might be helpful for anterior uveitis (when taken in combination with vitamin E),52 autism,40 easy bruising,29 minor injuries,41 protecting the liver in nonalcoholic steatohepatitis,154 speeding recovery from bedsores,37 treating female infertility (specifically, a condition called "luteal phase defect"),155 and preventing early rupture of the chorioamniotic membranes ("the water breaking") in pregnancy.156 Vitamin C might also improve the effectiveness of antibiotic treatment for Helicobacter pylori, the cause of most peptic ulcers.179 Given the limited nature of this research, however, further studies are needed to conclude that vitamin C is beneficial for these conditions.
Preliminary evidence suggests that cream containing vitamin C may improve the appearance of aging or sun-damaged skin.50,51,157 Inconsistent evidence suggests that oral or topical vitamin C, taken by itself or in combination with vitamin E, may also help protect the skin against sun damage.42-49
Double-blind studies of vitamin C for the following conditions have yielded mixed results: asthma,30, 178 male infertility,38,39 reducing the muscle soreness that typically develops after exercise,36,176,177 and hypertension.31-35,147,148 Note: Unexpectedly, one study found that a combination of vitamin C (500 mg daily) and grape seed oligomeric proanthocyanidins (OPCs) (1,000 mg daily) slightly increased blood pressure.158 Whether this was a fluke of statistics or a real combined effect remains unclear.
Limited and in some cases contradictory evidence suggests possible benefit in the prevention or treatment the following conditions:
- Allergies56-58
- Atrial fibrillation following coronary artery bypass grafting (CABG)182
- Bladder infections during pregnancy181
- Gallbladder disease (in women)53
- General anesthetics 171
- Glaucoma149
- Gout172
- Obesity173-175
- Vascular dementia54
Observational studies indicate that people with a higher intake of vitamin C have a lower incidence of macular degeneration, heart disease, cancer, and osteoarthritis.22-27,59-66 However, these findings do not indicate that vitamin C supplements will help prevent or treat these conditions. Observational studies are notoriously unreliable for showing the efficacy of treatments; only double-blind studies can do that. (For more information on why double-blind studies are so important, see Why Does This Database Rely on Double-blind Studies? ) Two large double-blind trials exploring the effectiveness of vitamin C for heart disease prevention—one in women at high risk 181 and the other in men at low risk— 183 failed to find any benefit at all.
Vitamin C has been proposed as a treatment for cancer, but this claim is very controversial, and there is as yet no scientifically meaningful evidence that it works.67-70
Massive doses of vitamin C have at times been popular among people with HIV infection based on highly preliminary evidence.71,72 An observational study linked high doses of vitamin C with slower progression to AIDS.73 However, a double-blind study of 49 people with HIV who took combined vitamins C and E or placebo for 3 months did not show any significant effects on the amount of HIV detected or the number of opportunistic infections.74 Furthermore, one study found that vitamin C at a dose of 1 g daily substantially reduced blood levels of the drug indinavir, a protease inhibitor used for the treatment of HIV infection.160 This could potentially cause the drug to fail.
A large, 8-year trial involving 11,545 physicians aged 50 years and older concluded that vitamins C and E, alone or in combination, did not lower the risk of cataracts.186
In a study of 80 women with Chlamydia trachomatis infection, adding vitamin C to doxycycline and triple sulfa vaginal cream reduced discharge and pain associated with intercourse.184
One substantial study failed to find vitamin C useful for improving high cholesterol.161
According to a double-blind, placebo-controlled study of 141 women with cervical dysplasia (early cervical cancer), vitamin C, taken at a dosage of 500 mg daily, does not help to reverse the dysplasia.75
Vitamin C also does not appear to be helpful for treating Raynaud’s phenomenon caused by scleroderma.150
What Is the Scientific Evidence for Vitamin C?
Colds
As the most famous of all natural treatments for the common cold, vitamin C has been subjected to irresponsible hype from both proponents and opponents. Enthusiasts claim that if you take vitamin C daily, you will never get sick, while critics of the treatment insist that vitamin C has no benefit at all.However, a cool-headed evaluation of the research indicates something in between. Numerous studies have found that vitamin C supplements taken at a dose of 1,000 mg daily or more throughout the cold season can modestly reduce symptoms of colds and help you get over a cold faster, but they do not generally help prevent colds.81,83,146,162,185
Reducing Cold Symptoms
Most studies on vitamin C have evaluated the potential benefits to be gained by taking vitamin C throughout the cold season. A review of 29 placebo-controlled trials involving over 11,000 people found that the use of vitamin C in this way can reduce symptoms and decrease the duration of colds.185 Other studies have found similar results.81,83 Many people use vitamin C for colds in a different way: they only begin taking it when cold symptoms start. Vitamin C is probably not effective when used in this way.83,140,185 One double-blind trial enrolled 400 individuals with new-onset cold symptoms, and divided them into four different daily vitamin C dosage groups: 30 mg daily (a dose lower than the minimum daily requirement, and used by the researchers as a placebo), 1,000 mg, 3,000 mg, or 3,000 mg with bioflavonoids.140 Participants were instructed to take the vitamin at the onset of symptoms and for the following 2 days. The results showed no difference in the duration or severity of cold symptoms among the groups. High-dose vitamin C taken at the onset of a cold, in other words, didn't help. A review of 7 randomized and non-randomized trials also found that taking vitamin C at the start of a cold did not offer any benefits.185 The bottom line: If you want to use vitamin C to take the edge off your colds, take the supplement throughout the winter.
There are numerous other natural treatments for the common cold as well, some of which may be more helpful than vitamin C. For more information, see the full Colds and Flus article.
Preventing Colds
Although two relatively recent studies suggest that regular use of vitamin C throughout the cold season can help prevent colds,167-168 they suffer from a variety of flaws, and most other studies have found little to no benefit along these lines.146 However, people who are truly vitamin C-deficient, such as elderly people in nursing homes, may show increased resistance to infection if they take vitamin C (or other nutrients).86
In addition, vitamin C might be helpful for preventing the respiratory infections that can follow heavy endurance exercise. Marathon running and similar forms of exertion can temporarily weaken the immune system, leading to infections. Vitamin C may be helpful. According to a double-blind, placebo-controlled study involving 92 runners, taking 600 mg of vitamin C for 21 days prior to a race, made a significant difference in the incidence of sickness afterwards.87 Within 2 weeks of the race, 68% of the runners taking placebo developed cold symptoms versus only 33% of those taking the vitamin C supplement. As part of the same study, non-runners of similar age and gender to those running were also given vitamin C or placebo. Interestingly, the supplement had no apparent effect on the incidence of upper respiratory infections in this group. Vitamin C seemed to be effective in this capacity only for those who exercised intensively!
Two other studies found that vitamin C could reduce the number of colds experienced by groups of people involved in rigorous exercise in extremely cold environments.88 One study involved 139 children attending a skiing camp in the Swiss Alps, while the other enrolled 56 military men engaged in a training exercise in northern Canada during the winter months. In both cases, the participants took either 1 g of vitamin C or placebo daily at the time their training program began. Cold symptoms were monitored for 1 to 2 weeks following training, and significant differences in favor of vitamin C were found.
However, one very large study of 674 US Marine recruits in basic training found no such benefit.89 The results showed no difference in the number of colds between the treatment and placebo groups.
What's the explanation for this discrepancy? There are many possibilities. Perhaps basic training in the Marines is significantly different from the other forms of exercise studied. Another point to consider is that the Marines didn't start taking vitamin C right at the beginning of training, but waited 3 weeks. The study also lasted a bit longer than the positive studies mentioned above, continuing for 2 months; maybe vitamin C is more effective at preventing colds in the short term. Of course, another possibility is that it doesn't really work. More research is needed to know for sure.
Preeclampsia Prevention
Preeclampsia is a dangerous complication of pregnancy that involves high blood pressure, swelling of the whole body, and improper kidney function. A double-blind, placebo-controlled study of 283 women at increased risk for preeclampsia found that supplementation with vitamin C (1,000 mg daily) and vitamin E (400 IU daily) significantly reduced the chances of developing this disease.96While this research is promising, larger studies are necessary to confirm whether vitamins C and E will actually work. The authors of this study point out that similarly sized studies found benefits with other treatments, such as aspirin, that later proved to be ineffective when large-scale studies were performed. Furthermore, keep in mind that we don't know whether such high dosages of these vitamins are absolutely safe for pregnant women.
Cancer Treatment
Cancer treatment is one of the more controversial proposed uses of vitamin C. An early study tested vitamin C in 1,100 terminally ill cancer patients. One hundred patients received 10,000 mg daily of vitamin C, while 1,000 other patients (the control group) received no treatment. Those taking the vitamin survived more than four times longer on average (210 days) than those in the control group (50 days).102 A large (1,826 subjects) follow-up study by the same researchers found a nearly doubled survival rate (343 days versus 180 days) in vitamin C-treated patients whose cancers were deemed "incurable," as compared to untreated controls.103 However, these studies were poorly designed, and other generally better-constructed studies have found no benefit of vitamin C in cancer.104,105 At the present time, vitamin C cannot be regarded as a proven treatment for cancer.Reflex Sympathetic Dystrophy
Reflex sympathetic dystrophy (RSD) is a set of symptoms that can develop in the legs, arms, feet, and hands after fractures and other injuries. Also called complex regional pain syndrome (CRPS), symptoms include changes in skin temperature and color over the affected area, accompanied by burning pain, sensitivity to touch, sweating, and limitation of range of motion. The cause of RSD is unknown, and the condition is very difficult to treat.Two studies performed by a single research group reported evidence that vitamin C can help prevent RSD after wrist fractures.110,180
In one of these, a total of 123 adults with wrist fractures were enrolled and followed for one year.110 All were given 500 mg of vitamin C or placebo daily for 50 days. The results showed significantly fewer cases of RSD in the treated group.
A subsequent study conducted by the same research group compared placebo against three dosages of vitamin C in 416 people suffering from wrist fracture.180 Again, treatment continued for 50 days. The results indicated that approximately 10% of those given placebo developed RSD, while less than 2% of those given either 500 mg or 1,500 mg of vitamin C daily did so. According to the statistical analysis used by the authors, this relative benefit was statistically significant. The 200 mg dose of vitamin C did appear to offer some protection too, but not as much.
Easy Bruising
A 2-month, double-blind study of 94 elderly people with marginal vitamin C deficiency found that vitamin C supplements decreased their tendency to bruise.111Hypertension (High Blood Pressure)
According to a 30-day, double-blind study of 39 individuals taking medications for hypertension, treatment with 500 mg of vitamin C daily can reduce blood pressure by about 10%.112 Smaller benefits were seen in studies of individuals with normal blood pressure or borderline hypertension.113,114,148 However, other studies have failed to find any significant blood pressure-lowering effect.115,116,147 This mixed evidence suggests, on balance, that if vitamin C does have any blood pressure-lowering effect, it is at most quite small.Maintaining the Effectiveness of Nitrate Drugs
Nitroglycerin and related nitrate medications are used for the treatment of angina. However, the effectiveness of these medications tends to diminish over time. According to a double-blind study of 48 individuals, use of vitamin C at a dose of 2,000 mg 3 times daily helped maintain the effectiveness of nitroglycerin.142 These findings are supported by other studies as well.143-145Note: Angina is too serious a disease for self-treatment. If you have angina, do not take vitamin C (or any other supplement) except on a physician’s advice.
Safety Issues
The US government has issued recommendations regarding tolerable upper intake levels (ULs) for vitamin C. The UL can be thought of as the highest daily intake over a prolonged time known to pose no risks to most members of a healthy population. The ULs for vitamin C are as follows:- Children
- 1-3 years: 400 mg
- 4-8 years: 650 mg
- 9-13 years: 1,200 mg
- Males and Females
- 14-18 years: 1,800 mg
- 19 years and older: 2,000 mg
- Pregnant Women
- 18 years old or younger: 1,800 mg
- 19 years and older: 2,000 mg
- Nursing Women
- 18 years old or younger: 1,800 mg
- 19 years and older: 2,000 mg
Concerns have been raised that long-term vitamin C treatment can cause kidney stones.129,130,163 However, in large-scale observational studies, individuals who consume large amounts of vitamin C have shown either no change or a decreased risk of kidney stone formation.131-133 Still, there may be certain individuals who are particularly at risk for vitamin C-induced kidney stones.134 People with a history of kidney stones and those with kidney failure who have a defect in vitamin C or oxalate metabolism should probably restrict vitamin C intake to approximately 100 mg daily. You should also avoid high-dose vitamin C if you have glucose-6-phosphate dehydrogenase deficiency, iron overload, or a history of intestinal surgery.
Vitamin C supplements increase absorption of iron.121-128,164 Since it isn’t good to get more iron than you need, individuals using iron supplements shouldn’t take vitamin C at the same time except under a physician’s supervision.
One study from the 1970s suggests that very high doses of vitamin C (3 g daily) might increase the levels of acetaminophen (such as Tylenol) in the body.135 This could potentially put you at higher risk for acetaminophen toxicity. This interaction is probably relatively unimportant when acetaminophen is taken in single doses for pain and fever, or for a few days during a cold. However, if you use acetaminophen daily or have kidney or liver problems, simultaneous use of high-dose vitamin C is probably not advisable.
Weak evidence suggests that vitamin C, when taken in high doses, might reduce the blood-thinning effects of warfarin (Coumadin) and heparin.136-139
As noted above, one study found that vitamin C at a dose of 1 g daily substantially reduced blood levels of the drug indinavir, a protease inhibitor used for the treatment of HIV infection.165
Heated disagreement exists regarding whether it is safe or appropriate to combine antioxidants such as vitamin C with standard chemotherapy drugs. The reasoning behind the concern is that some chemotherapy drugs may work in part by creating free radicals that destroy cancer cells, and antioxidants might interfere with this beneficial effect.76,166 However, there is no good evidence that antioxidants actually interfere with chemotherapy drugs, but there is growing evidence that they do not.77,79,151
The maximum safe dosages of vitamin C for people with severe liver or kidney disease have not been determined.
Interactions You Should Know About
If you are taking:- Aspirin, other anti-inflammatory drugs, or oral contraceptives: You may need more vitamin C.
- Acetaminophen (eg, Tylenol): The risk of liver damage from high doses of acetaminophen may be increased if you also take large doses of vitamin C.
- Warfarin (Coumadin) or heparin: High-dose vitamin C might reduce their effectiveness.
- Iron supplements: High-dose vitamin C can cause you to absorb too much iron. This is especially a problem for people with diseases that cause them to store too much iron.
- Medications in the nitrate family: Vitamin C may help maintain their effectiveness. Note: Angina is too serious a disease for self-treatment. If you have angina, do not take vitamin C (or any other supplement) except on a physician’s advice.
- Protease inhibitors for HIV: High-dose vitamin C may reduce their effectiveness.
- Cancer chemotherapy: Do not use vitamin C except on physician’s advice.
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