Consumers can never be sure about the quality of unregulated supplements, Lee says. She cites tryptophan tablets, a supplement that was popular and heavily marketed a few years ago. Impurities in some batches caused rheumatological and immunological complications, better known as eosinophilia myalgia syndrome. So far, studies of melatonin's effect on sleep have produced inconsistent results. Lee says melatonin may show some promise as an antioxidant, like vitamins C and E, which neutralize dangerous molecules known as free radicals.
Free radicals can mutate genes and eventually lead to cancer or cell death.
A number of studies investigating melatonin's value as an antioxidant are under way, but they have not yet produced conclusive results. However, more research on the long-term effects of melatonin is necessary. Individuals taking melatonin should be aware of potential risks.