Green’s Dictionary of Slang

(drugs) a small dose of LSD, usu. as placed on squares of blotting-paper.

1972		N.Y. Post 3 Jan. 5: Normally the drug produces hallucinations, but these microdots create fear, terror and suicidal tendencies.
1981		D.E. Miller Bk of Jargon 338: microdots: Weak LSD doses of approximately 100 micrograms each, usually in small blue beads.
1996		R. Rendell Keys to the Street 3281: He’d never been interested in acid, microdot, mushrooms or any of that stuff.
2001		ONDCP Street Terms 15: Microdot — LSD.