As with all medicines, different people can have different reactions. The most important factor here is dosage, which is usually responsible for most adverse reactions to melatonin - people taking too much! Consider that melatonin is a naturally occurring chemical within everyone's brain, so taking it is a "supplementation" rather than an additive. As it is naturally occurring, it is not an adverse reaction to melatonin, but an adverse reaction to the dosage. Start small and work your way up until positive effects are felt. If you only feel negative effects from it, simply stop. It won't kill you!