Green’s Dictionary of Slang

(drugs) any drug in pill form, e.g. amphetamines, barbiturates.

1966		J. Susann [bk title] Valley of the Dolls.
1969		Harper’s Mag. Oct. 65: Jacqueline Susann [...] has made the word ‘doll’ a synonym for pill.
1977		A. Brooke Last Toke 85: Redwood shelling out pills each time he entered the club, and Pecker [...] taking the doll with a loose smile.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Dolls. Barbiturates. Sometimes yellow dolls.
2001		ONDCP Street Terms 8: Dolls — Amphetamines; depressant; methylenedioxymethamphetamine (MDMA).