A tranquilizer is a medicine that calms someone down or makes them sleepy. Doctors use tranquilizers to help patients who feel extremely anxious or agitated, or to help someone relax before surgery.

Veterinarians also use tranquilizers on animals. When a zoo needs to move a large animal like a bear or elephant, they might use a tranquilizer dart that makes the animal fall asleep safely. This allows the vets to examine or transport the animal without anyone getting hurt. Wildlife biologists use tranquilizers too, putting animals briefly to sleep so they can attach tracking collars or check their health before the animal wakes up unharmed.