Green’s Dictionary of Slang

cap n.4

[abbr.]

1. [1920s+] (drugs) a capsule containing a narcotic, usu. heroin.

2. [1960s+] (drugs) a capsule of LSD.

3. [1960s+] (drugs) a capsule of any drug.

4. [1960s+] psilocybin/psilocin.

5. [1980s+] (drugs) crack cocaine.

6. [1990s+] (US prison) a measure of marijuana: as much as can be fitted into a small cap, e.g. of a toothpaste tube or a container of chapstick.

In phrases

bust a cap (v.)

[1950s–60s] (drugs) to inject a shot of heroin (which comes in capsule form).

cap out

[1970s] (US drugs) to collapse or fall asleep after the consumption of a drug.