Green’s Dictionary of Slang

1. depressants.

1969		R.R. Lingeman Drugs from A to Z (1970) 36: backwards [...] Tranquillizers.
1972		in Smith & Gay Heroin in Perspective.
2001		ONDCP Street Terms 2: Backwards — Depressants.

2. the state of mind specific to taking a barbiturate, esp. in response to an unpleasant experience while using LSD.

1971

E.E. Landy Underground Dict. (1972).