Vitamins: nutrients, hormones, drugs or poisons?

Conventional concepts of daily vitamin requirements may need revision. For example, recent studies on Vitamin C suggest that the recommended daily allowance, while it certainly prevents scurvy, may not saturate the body. Certain disease states and medications raise the daily requirement for specific vitamins. Most claims made by proponents of megavitamin therapy are unproven and without convincing theoretical bases. However, in at least two instances there is enough suggestive evidence to warrant more extensive trials. Although it seems that many vitamins can be given in large doses safely, Vitamins A and D and nicotinic acid, because of their toxicity, must be used with caution.