Green’s Dictionary of Slang

girl n.2

[the image of cocaine as a ‘feminine’ drug, the injecting of which gives a sexual thrill (although heroin, too, has that effect on some users), as opposed to heroin or boy n.2 (4a), a ‘masculine’ drug, i.e. one that ‘knocks you down’]
(drugs)

1. cocaine.

[US]Anslinger & Tompkins Traffic In Narcotics 309: the girl. Cocaine.
[US]C. Cooper Jr Scene (1996) 39: Leslie thought of copping — four girls and four boys. A speedball. [Ibid.] 60: Boy is the junkie’s name for heroin; they call cocaine girl because it gives’ em a sexual jab when they take a shot.
[US]D. Goines Dopefiend (1991) 159: He begged her out of a little girl – cocaine – and mixed it with his horse.
[US]E. Folb Runnin’ Down Some Lines 172: Cocaine had even fewer vernacular labels [...] girl, snow, coke [...] golden girl.
[US]Simon & Burns Corner (1998) 62: They called it ‘girl’ or ‘Jane’ or ‘Missy’ in feminine contrasts to ‘boy’ or ‘John’ or ‘Mister’ for king heroin.
[US]ONDCP Street Terms 10: Girl — Cocaine; Crack Cocaine.

2. heroin [may be a mis-reading].

[US]J.E. Schmidt Narcotics Lingo and Lore 68: Girls – Addiction narcotics.
[UK]Jagger & Richard ‘Dead Flowers’ [lyrics] I’ll be in my basement room / With a needle and a spoon / And another girl can take my pain away.
[US]ONDCP Street Terms 10: Girl — [...] heroin.

In compounds

girlfriend (n.)

see separate entry.