I take one Melatonin from time to time just as a sleep aid, especially on nights when I have to wake up super early.

So far I have experienced no side effects apart from the horrible inconvenience of waking up tired with an early alarm.

I would assume certain prescriptions will conflict with Melatonin use. In that case, please speak to your doctor. The only medication I'm on is for blood pressure --- I just recently accepted I have to seek help and therefore have no psychological diagnosis just yet. So in my case, it doesn't give me any nightmares but I would guess it may present a conflict with certain meds.