Green’s Dictionary of Slang

foil n.

(drugs) a quantity of drugs, e.g. amphetamine, heroin or cannabis, wrapped in foil ready for sale.

cited in Horman & Fox Drug Awareness (1970).
[US]E.E. Landy Underground Dict. (1972).
[Aus]R.G. Barratt ‘Kill Two Birds’ in What Do You Reckon (1997) [ebook] [A] plastic bag with about half-a-dozen foils in it. A few lousy grams of crappy leaf.
[Aus]Canberra Times 8 Aug. 3: Other details released yesterday show a foil of heroin, giving two hits, can be bought for $30. A starter kit costs $10.
[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 72/2: foil (also foilie) n. a bullet of cannabis, enough for about two or three cigarettes, wrapped in tinfoil.
Aus. Drug Federation Information Sheet 🌐 The capsules [of amphetamine] vary considerably in colour and are sometimes sold in commercial brand shells. They are packaged in ‘foils’ (aluminium foil), plastic bags or small balloons when sold on the streets.
[US]G. Pelecanos Drama City 191: I had a whole rack of foil in my pocket and took a felony charge.

In compounds

foil-face (n.)

(UK drugs) a heroin addict whose preferred method of ingestion is to inhale the fumes when the drug is heated on a piece of silver foil.

[UK]B. Hare Urban Grimshaw vii: Foil-face Person who smokes heroin from tinfoil, a practice often called chasing the dragon. [Ibid.] 41: ‘Dirty digger?’ ‘Nope.’ ‘Foil-face?’ ‘Nope.’.

In phrases

put on the foil (v.)

to inhale fumes from heroin heated on a piece of foil.

N. Dowd Slapshot [film script] Ned Braden: What are you doing? Jeff Hanson: Puttin’ on the foil! [...] Want some?