Green’s Dictionary of Slang

(US drugs) someone who volunteers to personally test illicit drugs.

2003

Microgram Bulletin XXXVI:11 249: 🌐 The tablets were seized during a probation check from a local drug user who is a so-called ‘Mikey’ (a volunteer ‘guinea pig’ who is willing to ‘test’ (by self-administration) illicit drugs and drug mixtures of virtually any type.