Green’s Dictionary of Slang

chemical n.

[its manufacture]

1. bad or adulterated narcotics.

[US](con. 1930s) Courtwright & Des Jarlais Addicts Who Survived 187: The Italians were selling shit, chemical, acid.

2. any chemical drug, as opposed to marijuana etc; often as chemicals.

[US]J. Blake letter 19 June in Joint (1972) 117: They [...] turn on with chemicals they primarily refer to as ‘stimulants’.
[US]M. Braly Shake Him Till He Rattles (1964) 106: ‘A few chemicals,’ Ann told him.
[US]L. Kramer Faggots 323: Why am I the only one who has such paranoia with my chemicals!
[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Chemicals 2. Illicit drugs.

3. (Aus. prison) toiletries, shampoo etc.

[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Chemicals. 1.That part of a prisoner’s buy-up, which relates to toiletries, hair shampoos, etc.

4. (drugs) crack cocaine.

[US]ONDCP Street Terms 5: Chemical — Crack Cocaine.

SE in slang uses

In compounds

chemical blonde (n.)

a dyed blonde.

[Can]J.C. Alvord ‘Sins of the Mothers’ in Canada Mthly Dec. 98/2: ‘I, the skirt dancer, the tights wearer, the chemical blonde’.
chemical head (n.)

(US black) hair that has been straightened through the application of a special mixture.

V. Matthews My Race 24: He still had his Chemical Head, with a black scarf tied around his head knotted in the front.