What are cotton balls for for shooting heroin?

Answer 1

Cotton balls are for many things, such as applying antiseptic, removing makeup, cleaning small areas of skin, stopping bleeding, etc. When getting an injection (such as with heroin), cotton balls are used for cleaning the area to be injected with an antiseptic and for stopping the tiny amount of bleeding once removing the needle.

Illegal heroin users also mix heroin with water, heat it, and strain it through a ball of cotton when filling the hypo. This is to help remove undissolved solids.