The Collaborative International Dictionary of English v.0.48:

Depressant \De*press"ant\, n. (Med.)
   An agent or remedy which lowers the vital powers.
   [1913 Webster]

WordNet (r) 3.0 (2006):

depressant
    adj 1: capable of depressing physiological or psychological
           activity or response by a chemical agent [ant:
           stimulative]
    n 1: a drug that reduces excitability and calms a person [syn:
         sedative, sedative drug, depressant, downer]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "depressant":
   Amytal, Amytal pill, Demerol, Dial, Dolophine, H, Luminal,
   Luminal pill, M, Mickey Finn, Nembutal, Nembutal pill, Seconal,
   Seconal pill, Sulfonal, Trional, Tuinal, Tuinal pill, alcohol,
   amobarbital sodium, analgesic, anodyne, barb, barbital,
   barbiturate, barbiturate pill, barbituric acid, belladonna, black,
   black stuff, bleak, blue, blue angel, blue devil, blue heaven,
   blue velvet, bromide, calmative, cheerless, chloral hydrate,
   codeine, codeine cough syrup, depressing, depressive, depressor,
   dismal, dispiriting, dolly, downer, dreary, goofball, hard stuff,
   heroin, hop, horse, hyoscyamine, hypnotic, junk, knockout drops,
   laudanum, liquor, lotus, meperidine, methadone, morphia, morphine,
   narcotic, opiate, opium, oppressive, pacifier, pain killer,
   paraldehyde, pentobarbital, phenobarbital, phenobarbital sodium,
   purple heart, quietener, rainbow, red, reserpine, scag,
   scopolamine, secobarbital sodium, sedative, shit, sleep-inducer,
   sleeper, sleeping draught, sleeping pill, smack, sodium bromide,
   sodium thiopental, somnifacient, soother, soothing syrup,
   soporific, tar, thalidomide, tranquilizer, turps, white stuff,
   yellow, yellow jacket