Green’s Dictionary of Slang

(drugs) barbiturates, amphetamines, morphine sulphate, all drugs available as pills.

1972		B. Rodgers Queens’ Vernacular 54: communist M and M’s (mid ’60s, fr M&M candies) ambars: pep-pills with a red candy-coated shell. Syn: M and M’s.
1980		E. Folb Runnin’ Down Some Lines 168: Other expressions likened Seconal to ‘candy’ [...]- M & Ms, gum drops, ju jus.
2001		ONDCP Street Terms 14: M&M — Depressants.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 112/1: M&M n. 1 a morphine sulphate tablet (MST).
2002		Greater Manchester Police Police and Schools Liaison Programme 🌐 Street names [...] M and M’s, MDMA, Shamrocks, X, XTC, Adam.