Green’s Dictionary of Slang

phenobarbital, phenobarbitone (a soporific drug best known by the US tradename Luminol).

1952		(con. 1948) G. Mandel Flee the Angry Strangers 442: ‘Any goofballs?’ she whispered [...] ‘Oney Phenobob.’.
1959		Kerouac letter Dec. in Charters II (1999) 240: As a doctor he’ll know that Phenos one pheno are a perfect antidote for two benzedrines ... taken with hot milk and honey.
1967		P. Thomas Down These Mean Streets (1970) 258: We also dealt for red capsules of phenos, two of which, with hot water, produce a forgetting high.
1971		E.E. Landy Underground Dict. (1972).
2001		ONDCP Street Terms 17: Phenos — Depressants.