Green’s Dictionary of Slang

(drugs) heroin.

1969		R.R. Lingeman Drugs from A to Z (1970) 115: H-caps heroin in gelatine capsules, each of which contains, roughly, 1 grain of cut (diluted) heroin. From ¼ ounce of pure heroin the wholesaler can fill up to 100 capsules for resale to the addict. In the 1940s and 1950s this was the favoured container for selling heroin, but it is less common today.
2001		ONDCP Street Terms 11: H Caps — Heroin.