Scientific Names: Ascorbic acid
Other Common Names: Ascorbate, cevitamic acid
Overall Safety: š Ā
Therapeutic Efficacy and Considerations:
Common Cold: š Many studies have examined vitamin C for both prevention and treatment of the common cold. The most recent meta-analysis determined that large doses on a daily basis do not reduce the incidence of colds or upper-respiratory tract infections, but that use of high doses at the onset of symptoms may provide a slight benefit to reduce duration. A recent high-quality trial for treatment compared 3 different doses to a āplaceboā dose of 30 mg/day and found no difference between any group. At this time, the evidence does not support use of high doses for either the prevention or treatment of colds or upper respiratory symptoms. Despite the lack of evidential support, many patients will be taking vitamin C for this reason. They should be counseled to take smaller (100-250 mg) doses every 4-6 hours, rather than larger doses only once daily. This allows serum vitamin C to remain at higher levels more consistently to optimize any chance of benefit.
Hyperlipidemia: š Four controlled trials for use in hyperlipidemia have demonstrated very mixed results. More research is warranted to determine true efficacy and if there are certain populations who do or do not derive benefit from use. Currently, the evidence does not support use for this indication. Patients who do choose to use vitamin C for this purpose do not need to be discouraged. Counseling should include appropriate dose of 1000 mg daily and that this therapy should only be used in addition to lifestyle changes.
Hypertension: š Only one of five controlled trials noted any benefit of vitamin C for treatment of hypertension and that trial was in diabetic patients. More research is needed to assess possible benefits in that patient population. Current evidence does not support use for this indication in any patient population.
CHD in Smokers: š Only one small pilot trial using a single large IV of vitamin C has been conducted. Improvement in coronary flow reserve were observed. These results warrant more research but are insufficient to support use of high dose oral supplementation for this purpose. Smokers do have increased vitamin C requirements and should be counseled to take at least 100 mg of vitamin C daily in addition to a multivitamin supplement.
Chemistry/Pharmacology:
Vitamin C is a water-soluble essential vitamin; although many animals produce vitamin C, humans must obtain it via food sources. Fresh vegetables and fruits, especially citrus fruits, are good sources. Vitamin C is involved in many body processes, primarily as a reducing agent, including production of thyroxin, neurotransmitters, collagen, and carnitine and metabolism of tyrosine, folic acid, histamine, and others. It enhances absorption of iron and some other minerals. It is well-known that vitamin C requirements are increased in patients with severe wounds and trauma, burns, high stress, infection and fever, and pregnancy/lactation; smokers also have increased requirements. Exogenous administration of vitamin C does seem to have some beneficial increases on immune functions such as phagocytic activity, T-lymphocytes, and increasing mobility of leukocytes. There may be increases in interferon and antibody production. Other immune benefits may be associated with antioxidant activity. In cardiovascular disease, vitamin C may help with endothelial function and vascular function by maintaining nitric oxideās vasodilatory effects.
Drug Interactions:
At high doses, will decrease blood levels of fluphenazine and substantially inhibit ACETAMINOPHEN elimination (doses >3 g/day) which could lead to toxicity. Doses >1 g/day may interfere with disulfiram activity. Large doses decrease excretion of salicylates; this is probably only clinically significant for persons taking large doses of aspirin or other salicylates on a daily basis. Vitamin C may decrease the effectiveness of some chemotherapies and increase effectiveness of others; specific information for each drug in a regimen must be researched before use. Indinavir levels will be decreased (one study noted 14%) to an extent that may not be clinically significant; until more information is known, it would be best to keep doses under 1 g/day with indinavir or other inhibitors.
Contraindications/Precautions:
Use with caution in patients with hemochromatosis (due to increased iron absorption), patients prone to developing oxalate kidney stones. Sickle cell disease patients should avoid high doses that may lower blood pH, due to the possibility of precipitating a sickle cell crisis.
Adverse Effects:
Doses greater than 1 g/day are associated with osmotic diarrhea. Response is very individual, and many patients can take up to 6-10 g/day before experiencing diarrhea. Chewable vitamin C tablets have been associated with increased dental caries and erosion; these should not be used long-term.
