Viral Upper Respiratory Infections (Colds and Influenza) and Vitamin C

Vitamin C has received a lot of attention for its potential to help prevent and treat infections like those that cause colds and the flu. Research demonstrates that, while vitamin C may mildly reduce symptoms of colds when they occur, it probably does not help prevent colds.

Vitamin C supports immune function, but the exact effect of vitamin C on colds and flu is not clear. Vitamin C serves numerous functions in the body, from producing collagen to working as an antioxidant.

Read more details about Vitamin C.

Treating Colds

Numerous studies have found that vitamin C supplements taken at a dose of 1,000 mg or more daily can reduce the severity cold symptoms and shorten their duration. However, the effect is modest at best.^{50,51,100,172} In addition, at least one study suggests that vitamin C can enhance the effect of standard cold treatments, such as acetaminophen.¹⁰⁹ According to most of these studies, using vitamin C throughout the cold season, rather than only occasionally, appears to be beneficial. For example, a review of 29 placebo-controlled trials involving almost 1,000 episodes of illness concluded that taking at least 2,000 mg per day preventively seems to result in shorter and less severe colds when they occur.¹⁷² However, high doses of vitamin C does not appear capable of decreasing the number of colds experienced during a season.

But, many people use vitamin C for colds in a different way: they only begin taking it after cold symptoms first appear. In a review of 7 randomized and non-randomized trials, researchers found that this approach—taking high doses of vitamin C (eg, 2,000 - 4,000 mg) at the first sign of illness—does not seem to affect the cold's severity or duration.¹⁷²

Preventing Colds

Although some studies suggest that regular use of vitamin C throughout the cold season can help prevent colds,^100,149 most other studies have found little to no benefit along these lines.⁹⁹

Vitamin C has shown a bit more promise for prevention of one type of cold, the post-marathon sniffle. These are colds that develop after endurance exercise; use of vitamin C before and during competition may help keep you cold-free afterwards.^54,55 In addition, vitamin C seems to help prevent respiratory infections among individuals who are actually deficient in the vitamin.⁵⁶

Vitamin C is an essential nutrient that must be obtained from food or supplements; the body cannot manufacture it. 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 greens, 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.

Many nutritional experts recommend a total of 500 mg of vitamin C daily. This dose is almost undoubtedly safe. Therapeutic doses such as those used in the research on colds and flu tend to be in the 1,000-2,00 mg range.

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

However, even within the safe intake range for vitamin C, some individuals may develop diarrhea. This side effect will likely go away with continued use of vitamin C, but you might have to cut down your dosage for a while and then gradually build up again.

Concerns have been raised that long-term vitamin C treatment can cause kidney stones . ¹ 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. ^{2 3} Still, there may be certain individuals who are particularly at risk for vitamin C-induced kidney stones. ⁴ 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 . ⁵ 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. ⁶ 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 . ^{7 8 9} 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. ¹⁰ 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. ¹¹ However, there is no good evidence that antioxidants actually interfere with chemotherapy drugs, but there is growing evidence that they do not. ¹² 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.

1. Auer BL, Auer D, Rodgers AL. Relative hyperoxaluria, crystalluria, and hematuria after mega-dose ingestion of vitamin C. Eur J Clin Invest. 1998;28:695-700.
2. Curhan GC, Willett WC, Speizer FE, et al. Intake of vitamins B 6 and C and the risk of kidney stones in women. Am Soc Nephrol. 1999;10:840-845.
3. Curhan GC. A prospective study of the intake of vitamin C and vitamin B 6 and the risk of kidney stones in men. J Urol. 1996;155:1847-1851.
4. Auer BL, Auer D, Rodgers AL. Relative hyperoxaluria, crystalluria, and hematuria after mega-dose ingestion of vitamin C. Eur J Clin Invest. 1998;28:695-700.
5. Maskos Z, Koppenol WH. Oxyradicals and multivitamin tablets. Free Radic Biol Med. 11(6):609-10.
6. Houston JB, Levy G. Drug biotransformation interactions in man VI: acetaminophen and ascorbic acid. J Pharm Sci. 65(8):1218-21.
7. Owen CA Jr, Tyce GM, Flock EV, McCall JT. Heparin-ascorbic acid antagonism. Mayo Clin Proc. 45(2):140-5.
8. Rosenthal G. Interaction of ascorbic acid and warfarin [letter]. JAMA. 1971;215:1671.
9. Harris JE. Interaction of dietary factors with oral anticoagulants: review and applications. J Am Diet Assoc. 95(5):580-4.
10. Slain D, Ansden J, Khakoo R, et al. Effects of high-dose vitamin C on the steady state pharmacokinetics of the protease inhibitor Indinavir in healthy volunteers. Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) Meeting; Sept 13-17, 2003; Chicago, IL. Poster A-1610.
11. Labriola D, Livingston R. Possible interactions between dietary antioxidants and chemotherapy. Oncology. 1999;13:1003-1012.
12. Weijl NI, Cleton FJ, Osanto S. Free radicals and antioxidants in chemotherapy-induced toxicity. Cancer Treat Rev. 23(4):209-40.

50. Hemila H. Does vitamin C alleviate the symptoms of the common cold?—A review of current evidence. Scand J Infect Dis. 1994;26:1-6.
51. Hemila H. Vitamin C and the common cold. Br J Nutr. 1992;67:3-16.
52. Hemila H. Vitamin C supplementation and common cold symptoms: factors affecting the magnitude of the benefit. Med Hypotheses. 1999;52:171-178.
53. Hemila H. Vitamin C supplementation and common cold symptoms: factors affecting the magnitude of the benefit. Med Hypotheses. 1999;52:171-178.
54. Peters EM, Goetzsche JM, Grobbelaar B, et al. Vitamin C supplementation reduces the incidence of postrace symptoms of upper-respiratory-tract infection in ultramarathon runners. Am J Clin Nutr. 1993;57:170-174.
55. Hemila H. Vitamin C and common cold incidence: a review of studies with subjects under heavy physical stress. Int J Sports Med. 1996;17:379-383.
56. Hemila H. Vitamin C intake and susceptibility to the common cold. Br J Nutr. 1997;77:59-72.
99. Hemila H. Vitamin C intake and susceptibility to the common cold. Br J Nutr. 1997;77:59-72.
100. Van Straten M, Josling P. Preventing the common cold with a vitamin C supplement: a double-blind, placebo-controlled survey. Adv Ther. 2002;19:151-159.
149. Sasazuki S, Sasaki S, Tsubono Y, et al. Effect of vitamin C on common cold: randomized controlled trial. Eur J Clin Nutr. 2005 Aug 24. [Epub ahead of print]
172. Hemila H, Chalker E, Douglas B. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2010;(3):CD000980.