I've taken it on occasion, and the only side-effect I noticed was a slight headache in the morning, but I have allergies & often wake up with a headache, so it could have been the allergies, not the melatonin.
I use black cohosh for post-menopausal symptoms, and the formula I'm using now contains valerian root. It helps me sleep, and I've noticed no side effects at all. As a matter of fact, since I switched to this formula, not only do I fall asleep easier (no "knock out" effect -- just an ease into sleep), but I stay asleep, whereas before, I was up at least once during the night, sometimes for several hours.
The only time I take benadryl is when my allergies are unbearable. It is habit forming, and it can be abused.
Peace & Love,
Sugah