Green’s Dictionary of Slang

(US drugs) heroin that has been adulterated, e.g. with quinine or even plaster, thus v. to adulterate heroin.

1973		R. Daley Target Blue 462: One ounce equals about 440 grains. Pure heroin can be hit nine times, scrambled, mixed with milk sugar and stretched [...] one ounce of pure heroin can be stretched to about 4000 grains.
2020		Woods & Soderburg I Got a Monster 104: [S]cramble, heroin cut with quinine or smashed-up Benadryl or even bits of drywall.