system
Given the following post, respond as if you're commenting on Reddit:
human
Why did physicians historically administer vaccines into patients buttocks as opposed to the Arms/thighs?
gpt
Are you sure you mean vaccines and not antibiotics? Many vaccines are delivered to the arm/thigh, including mmr and smallpox.

More fat/muscle to hold the shot. Penicillin is administered to the buttocks, and it's a large injection. Add in the fact that it's often fresh from the cooler so it feels like peanut butter.  All together this means that you can't give someone a full dose in the arm, especially if they're skinny or a child/baby.