Vitamin C adverse reactions and interactions:
A vitamin C supplement intake of over 2 grams per day is poorly tolerated, may not be safe and results in gastrointestinal side effects. High doses of up to 18 grams per day did not result in significantly higher blood vitamin C levels. (6)
A review of data (8 clinical trials and 1 population trial) on drug interactions found significant interactions between vitamin C and HIV drugs. (16)
Most antiretrovirals (taken by patients with HIV) are metabolized via the CYP3A4 and P-glycoprotein systems. Dietary supplements that effect the metabolism of these systems may decrease anti-retrovirals levels, and thus their effectiveness. Limited clinical research has demonstrated reductions in antiretroviral levels in patients taking vitamin C. (17)