<p>Cholestyramine, Colestipol, Neomycin cause decreased absorption of &beta; carotene. Circulating vitamin C levels have been shown to be reduced during prolonged administration of oral contraceptives containing Oestrogen, Tetracycline and Aspirin. The decrease in vitamin C level may be due to drug induced impaired absorption or increased utilization of the vitamin for drug metabolism. Vitamin E may enhance the anticoagulant activity of anticoagulant drugs. High doses of vitamin E can impair intestinal absorption of vitamins A and K.</p>