Over-the-counter medications must be safe, right? And since melatonin can be bought at any pharmacy or natural food store in the United States, it must be safe, too? It would seem logical that anything that is legal and easy to buy would be healthy, but that is not necessarily true.

Because melatonin is found in certain foods, the Food and Drug Administration considers it to be a dietary supplement instead of a drug. However, since melatonin has not been studied very extensively, it is still unknown if it causes long-term side effects.

While legally speaking melatonin is a dietary supplement, biologically speaking it is a hormone. Other types of hormones include estrogen and testosterone. Hormones are known to have major effects on the human body. Only one other hormone can be sold over the counter in the United States. All other hormones require a...