Green’s Dictionary of Slang

twenty-five n.¹

(drugs) LSD.

1969		R.R. Lingeman Drugs from A to Z (1970).
1973		C. Gaines Stay Hungry 146: I’ve done it all, man, glue, mushrooms, twenty-five, turps, Nembutal, THC, hash ... I been weird every way there is.
1981		D.E. Miller Bk of Jargon 339: 25: LSD.
2001		ONDCP Street Terms 22: Twenty-five — LSD.