Green’s Dictionary of Slang

come on v.2

[SE come on, to begin]

1. [1930s+] (drugs) for withdrawal symptoms to start affecting a narcotics addict.

2. [1940s+] (drugs) for a drug to begin affecting its user, esp. of a hallucinogen (which takes a short time to enter the bloodstream and hit the brain).

3. [1980s+] of a woman, to start menstruating.