WordNet (r) 3.0 (2006):

sleeping pill
    n 1: a soporific drug in the form of a pill (or tablet or
         capsule) [syn: sleeping pill, sleeping tablet,
         sleeping capsule, sleeping draught]

Moby Thesaurus II by Grady Ward, 1.0:

88 Moby Thesaurus words for "sleeping pill":
   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,
   anesthetic, 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, dope, downer,
   drug, goofball, hard stuff, heroin, hop, horse, hypnotic, junk,
   knockout drop, knockout drops, laudanum, liquor, lotus, meperidine,
   methadone, morphia, morphine, narcotic, opiate, opium, pacifier,
   pain killer, pain-killer, paregoric, pen yan, phenobarbital,
   phenobarbital sodium, purple heart, quietener, rainbow, red, scag,
   secobarbital sodium, sedative, shit, sleep-inducer, sleeper,
   sleeping draught, smack, sodium thiopental, somnifacient, soother,
   soothing syrup, soporific, tar, tranquilizer, turps, white stuff,
   yellow, yellow jacket