ZzzQuil helps you fall asleep by introducing the ingredient Diphenhydramine HCL into your system. Diphenhydramine HCL is an antihistamine that has been shown to make you feel sleepy and help you relax with no lasting side effects. Unlike a more powerful prescription sleep aid, ZzzQuil is gentle and won’t force you to fall asleep; it will only help calm, soothe and guide you into sleep.