ZzzQuil is an OTC (over-the-counter) antihistamine used to alleviate the symptoms of the common cold, allergies and hay fever to help you sleep better. This nighttime sleep aid is intended to be used occasionally. If you are having difficulty falling asleep, ZzzQuil reduces the time it takes to fall asleep. ZzzQuil is an over-the-counter nighttime sleep aid. The active ingredient in ZzzQuil is diphenhydramine, which is also the active ingredient in the allergy medication Benadryl.