Green’s Dictionary of Slang

doll n.2

[popularized and apparently coined by the book/film Valley of the Dolls (1968) by Jacqueline Susann]

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

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