Tranquillizer
/ˈtræŋkwəlaɪzər/
Definition 1 of 2
Pro Tip 1/3Medical Use
Used in healthcare to describe medications for anxiety or insomnia. 
The doctor prescribed a tranquillizer for her severe anxiety.
Pro Tip 2/3Veterinary Setting
Often used by vets to sedate animals for treatment or transport. 
The lion was given a tranquillizer before being moved to another zoo.
Pro Tip 3/3Calm Emotions
May also refer to reducing psychological stress or excitement. 
After the traumatic event, he needed a tranquillizer to sleep.