An acniform eruption or worsening of acne vulgaris may occur with supplementation. Magnesium and B6 deficiency produce comparable clinical disorders and a deficiency of one nutrient may be treated with either nutrient in the conditions that have been studied. Supplementation may cause an acniform eruption or exacerbate acne vulgaris. Oral or sublingual administration apparently produces negligible blood levels compared to IM injection. High doses of folic acid may reduce B12 levels.
Diarrhea is the main side effect of high dosages.
Vitamin C enhances iron absorption and is contraindicated in cases of iron overload. Sudden discontinuation of high dosages can cause rebound scurvy so that supplementation should be tapered gradually over several days to weeks. Vitamin C enhances aluminum absorption and should not be taken with aluminum-containing substances especially in the presence of renal insufficiency. Experimental studies show that vitamin C with citrus bioflavonoids is better absorbed and stays in the body longer than the equivalent dose of vitamin C alone. Excessive intake may cause hypercalcemia, due to increased intestinal absorption of calcium, leading to calcium phosphate deposition in the kidney with nephrocalcinosis and decreased kidney function.