"Smack" is the "common" name for

Heroin or di-acetyl-morphine is about two times more potent than morphine in injectable form. Apa from the parenteral mode of administration, heroin can also be smoked or 'chased' ( chasing the dragon), often in an impure form (called ' smack' or ' brown sugar' in India). Heroin is more addicting than morphine and can cause dependence even after a sho period of exposure. Ref: A Sho Textbook of Psychiatry, Niraj Ahuja, 7th Edition, pg. no. 43