Why do injections usually spank the butt?

Spanking on the butt is a kind of intramuscular injection with a medium speed of action. It is to inject the drug into the muscle so that the small blood vessels in the muscle can slowly absorb it and take effect. Common sites for intramuscular injection include the arms, buttocks, and buttocks.

Muscles are often chosen because they are thicker and less likely to penetrate deeper blood vessels. There are many nerves distributed around the buttocks. To avoid hurting the nerves, the injections in the buttocks are injected into the outer 1/4 of the buttocks on one side.