Research is tempering the perception of the immune-boosting potential of vitamin C supplements.
Vitamin C, also known as ascorbic acid, is a water-soluble vitamin found in many multivitamin supplements and single-entity nutritional supplements. Vitamin C has an important role in the formation of collagen, carnitine, amino acids, and hormones; is an essential component in the healing of wounds and burns; and aids in the absorption of iron.1,2
Moreover, vitamin C is classified as an antioxidant, and many clinical studies report that megadoses (500 to 1000 mg/day) of ascorbic acid may prevent, or shorten the duration of, the common cold.1,2
While vitamin C is FDA approved for the treatment of scurvy, which is rare in the United States, and to acidify the urine, vitamin C is most often used by consumers to boost the immune system.2-4
Some clinical studies suggest that vitamin C may have mild in vitro antiviral activity against the influenza virus; however, clinical research to validate this is lacking.5,6
Results from a 2010 analysis of 29 clinical trials with an estimated 11,000 subjects reported that taking vitamin C routinely (at least 0.2 g/day) did not decrease the likelihood of getting a cold.3,4
However, the use of vitamin C supplements was associated with a slight decrease in the severity of cold symptoms and the duration of the common cold, with some studies reporting a reduction of 8% in adults and 14% in children.3,4
A review published in 2009 presenting research on the effects of vitamins and minerals for treating colds concluded that supplementation with vitamin C does have some potential benefits for treating the common cold; however, because there are only a few therapeutic trials, more research is needed to ascertain the effective dosages and the treatment guidelines.4
Many clinical studies suggest that vitamin C is most effective for reducing the duration of upper respiratory tract infection symptoms in children and healthy adults, but it does not appear to be as useful once a patient exhibits symptoms.3,4
In general, vitamin C is considered to be safe, but adverse effects have been reported when this supplement is taken in megadoses.4
According to the National Institutes of Health (NIH), although vitamin C has long been utilized as a popular remedy for the common cold, research shows that overall, vitamin C supplements do not reduce the risk of getting the common cold.7
When ingested in megadoses of 4 or more g per day, vitamin C may cause diarrhea, nausea, stomach cramps, and nephrolithiasis; therefore, megadoses of the vitamin are typically not recommended.2,4
Other potential adverse effects include headache, dizziness, flushing, and fatigue.8
To prevent the development of kidney stones, patients should be advised to drink a full glass of water when taking vitamin C.2
Patients with diabetes, a history of recurring renal calculi, and/or renal dysfunction should be advised to avoid prolonged use of megadoses of vitamin C supplements.2
Although most individuals in the United States obtain adequate vitamin C via dietary means, the NIH reports that certain patient populations may be at greater risk of not obtaining sufficient recommended daily allowances (Table 1; Table 2).9
In addition, the NIH reports that because vitamin C is an antioxidant and has a role in the immune process, various clinical studies are investigating the possible roles of this vitamin, such as cancer prevention and treatment, age-related macular degeneration, cataracts, and cardiovascular disease.7,8
Nonprescription Vitamin C Supplements
Nonprescription vitamin C dietary supplements typically contain ascorbic acid, which is considered to have bioavailability equivalent to that of naturally occurring ascorbic acid in foods such as orange juice and broccoli, but some supplements contain other forms, such as sodium ascorbate, calcium ascorbate, other mineral ascorbates, and ascorbic acid with bioflavonoids (Table 3).7,8
Adult multivitamin supplements typically contain doses of 60 to 100 mg, which are considered to be adequate if supplements are required.2
Vitamin C is available in several dosage formulations, including capsule, tablet, lozenge, syrup, chewable tablet, effervescent tablet, oral disintegrating tablet, and gummy. According to the Handbook of Nonprescription Drugs
, the recommended daily upper limit of vitamin C is 2 g.2
Ideally, patients should obtain their vitamin C through dietary means; however, some patients elect to use supplements. During counseling, patients electing to use vitamin C supplements should be reminded to take only the recommended dosage unless otherwise directed by their primary health care provider. Pharmacists should also screen patients for potential drug interactions, contraindications, and therapeutic duplications. For example, vitamin C supplements may interact with chemotherapeutic agents, so patients should be advised to talk to their primary health care provider prior to using these supplements.9-11
Some research indicates that vitamin C supplements may also interact with HMG-CoA reductase inhibitors (statins) when taken in combination with other antioxidants, such as beta carotene and vitamin E.9-11
This combination may decrease the effectiveness of statins; however, it is not known whether vitamin C alone affects the effectiveness of this drug class.9-11
In addition, megadoses of vitamin C may decrease the effectiveness of some agents, such as protease inhibitors, warfarin, estrogens, and niacin.9-11
Ms. Terrie is a clinical pharmacy writer based in Haymarket, Virginia.
Vitamin C. Merck Manual for Healthcare Professionals Online Edition. www.merckmanuals.com/professional/nutritional_disorders/vitamin_deficiency_dependency_and_toxicity/vitamin_c.html. Accessed January 3, 2014.
Huckleberry Y, Rollins C. Essential and conditionally essential nutrients. In: Krinsky D, Berardi R, Ferreri S, et al, eds. Handbook of Nonprescription Drugs. 17th ed. Washington, DC: American Pharmacists Association; 2012.
Kemper K, et al. Complementary and alternative medicine therapies for cold and flu season: what is the science? Medscape website. www.medscape.com/viewarticle/711485_3. Accessed January 3, 2014.
The flu, the common cold, and complementary health approaches. National Institutes of Health National Center for Complementary and Alternative Medicine website. http://nccam.nih.gov/health/flu/ataglance.htm. Accessed January 3, 2014.
Furuya A, Uozaki M, Yamasaki H, Arakawa T, Arita M, Koyama AH. Antiviral effects of ascorbic acid and dehydroascorbic acids in vitro. Int J Mol Med. 2008;22:541-545.
Jariwalla RJ, Roomi MW, Gangapurkar B, Kalinovsky T, Niedzwiecki A, Rath M. Suppression of influenza A virus nuclear antigen production and neuraminidase activity by a nutrient mixture containing ascorbic acid, green tea extract and amino acids. Biofactors. 2007;31:1-15.
Vitamin C. National Institutes of Health website. http://ods.od.nih.gov/factsheets/VitaminC-QuickFacts/. Accessed January 4, 2014.
Ascorbic acid. Medscape Drug Information. http://reference.medscape.com/drug/cenolate-vitamin-c-ascorbic-acid-344416#91. Accessed January 3, 2014.
Dietary supplement fact sheet: vitamin C. National Institutes of Health Office of Dietary Supplements website. http://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/. Accessed January 3, 2014.
Possible interactions with vitamin C. University of Maryland Medical Center website. http://umm.edu/health/medical/altmed/supplement-interaction/possible-interactions-with-vitamin-c-ascorbic-acid. Accessed January 3, 2014.
Vitamin C. Medline Plus website. www.nlm.nih.gov/medlineplus/druginfo/natural/1001.html#DrugInteractions. Accessed January 4, 2014.
Institute of Medicine; Food and Nutrition Board. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington, DC: National Academies Press; 2000.