The Collaborative International Dictionary of English v.0.48:

Tranquilizer \Tran"quil*i`zer\, Tranquillizer \Tran"quil*li`zer\
,  n.
   One who, or that which, tranquilizes.
   [1913 Webster] Tranquilizing

WordNet (r) 3.0 (2006):

tranquilizer
    n 1: a drug used to reduce stress or tension without reducing
         mental clarity [syn: tranquilizer, tranquillizer,
         tranquilliser, antianxiety agent, ataractic drug,
         ataractic agent, ataractic]

Moby Thesaurus II by Grady Ward, 1.0:

159 Moby Thesaurus words for "tranquilizer":
   Amytal, Amytal pill, DET, DMT, Demerol, Dolophine, H, LSD, Librium,
   Luminal, Luminal pill, M, Mary Jane, Mickey Finn, Nembutal,
   Nembutal pill, STP, Seconal, Seconal pill, Serpasil, THC,
   Thorazine, Tuinal, Tuinal pill, Valium, acid, alcohol, alleviative,
   alleviator, amobarbital sodium, analgesic, anesthetic, anodyne,
   antidepressant, assuager, ataractic, balm, barb, barbiturate,
   barbiturate pill, black stuff, blue, blue angel, blue devil,
   blue heaven, blue velvet, calmative, chloral hydrate, codeine,
   codeine cough syrup, cushion, depressant, depressor,
   diethyltryptamine, dimethyltryptamine, dolly, dolorifuge, dope,
   downer, drug, gage, ganja, goofball, grass, hallucinogen,
   hard stuff, hash, hashish, hay, hemp, heroin, hop, horse, hypnotic,
   joint, junk, kava, knockout drop, knockout drops, laudanum,
   lenitive, liquor, lotus, marijuana, meperidine, mescal,
   mescal bean, mescal button, mescaline, methadone,
   mind-altering drug, mind-blowing drug, mind-expanding drug,
   mitigator, moderator, modulator, mollifier, morning glory seeds,
   morphia, morphine, narcotic, opiate, opium, pacificator, pacifier,
   pain killer, pain-killer, palliative, paregoric, peacemaker,
   pen yan, peyote, phenobarbital, phenobarbital sodium, pot,
   psilocin, psilocybin, psychedelic, psychic energizer,
   psychoactive drug, psychochemical, psychotomimetic, purple heart,
   quietener, rainbow, rauwolfia, red, reefer, reserpine,
   restraining hand, roach, salve, scag, secobarbital sodium,
   sedative, shit, shock absorber, sleep-inducer, sleeper,
   sleeping draught, sleeping pill, smack, sodium thiopental,
   somnifacient, soother, soothing syrup, soporific, stabilizer,
   stick, tar, tea, temperer, thalidomide, turps, weed, white stuff,
   wiser head, yellow, yellow jacket