The Collaborative International Dictionary of English v.0.48:

heroin \her"o*in\ (h[~e]r"[-o]*[i^]n), n. (Chem.)
   a morphine derivative, diacetyl morphine, used to relieve
   severe pain and as a sedative. It is highly addictive, and
   its use is strictly controlled in the U.S. by federal law. It
   is a popular strong narcotic drug of abuse, in part because
   it is more soluble than morphine. It is sometimes included as
   one of the components of Brompton's mixture, used to control
   pain in terminallly ill patients.

   Syn: diacetyl morphine, H, horse, junk, scag, shit, smack.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

heroin
    n 1: a narcotic that is considered a hard drug; a highly
         addictive morphine derivative; intravenous injection
         provides the fastest and most intense rush [syn: heroin,
         diacetylmorphine]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "heroin":
   Amytal, Amytal pill, Demerol, Dolophine, H, Luminal, Luminal pill,
   M, Mickey Finn, Nembutal, Nembutal pill, Seconal, Seconal pill,
   Tuinal, Tuinal pill, alcohol, amobarbital sodium, analgesic,
   anodyne, barb, barbiturate, barbiturate pill, black stuff, blue,
   blue angel, blue devil, blue heaven, blue velvet, calmative,
   chloral hydrate, codeine, codeine cough syrup, depressant,
   depressor, dolly, downer, goofball, hard stuff, hop, horse,
   hypnotic, junk, knockout drops, laudanum, liquor, lotus,
   meperidine, methadone, morphia, morphine, narcotic, opiate, opium,
   pacifier, pain killer, paregoric, pen yan, phenobarbital,
   phenobarbital sodium, purple heart, quietener, rainbow, red, scag,
   secobarbital sodium, sedative, shit, sleep-inducer, sleeper,
   sleeping draught, sleeping pill, smack, sodium thiopental,
   somnifacient, soother, soothing syrup, soporific, tar,
   tranquilizer, turps, white stuff, yellow, yellow jacket