I use melatonin when I travel internationally. I am a former nurse, and I do work related to pharmaceuticals and the U.S. Food and Drug Administration. Periodically I check the medical literature for reports of side effects and have not seen anything of concern to me. I also check the FDA's reports of adverse events, and although nutritional supplements are not regulated by FDA, they do get reports on those types of substances (even more so after the ephedra problems of last few yrs.) But I have not seen anything I considered a problem that would make me think twice about using it. If one was going to take it longer term, I would hesitate as these suplements are not regulated for safety as well as effectiveness, But I only use one dose per trip, that is all I need. I buy melatonin that is produced from a reputable manufacturer, as these nutritional supplements are not regulated for purity either; that is probably my biggest concern about these things. For example the problem several years ago with L-tryptophan that had impurities in it; it gave people a muscular disease. But again, that was longer term use, and the problem was limited to one manufacturer (or possible a few, they got the L-tryptophan from the same contaminated source though.) So I do think about these things, but the melatonin works so well, and I stay w/the reputable manufacturer's version, and I only need one dose, so I continue to use it once a year or so when I am traveling internationally.