The Collaborative International Dictionary of English v.0.48:

downer \downer\ n.
   a drug that reduces excitability and calms a person.
   [Colloq.]

   Syn: sedative, depressant.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

downer
    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:

169 Moby Thesaurus words for "downer":
   Amytal, Amytal pill, Demerol, Dolophine, H, Luminal, Luminal pill,
   M, Mickey Finn, Nembutal, Nembutal pill, Schmerz, Seconal,
   Seconal pill, Slough of Despond, Tuinal, Tuinal pill,
   adverse circumstances, adversity, affliction, aggravation, alcohol,
   amobarbital sodium, analgesic, annoyance, anodyne, bad news, barb,
   barbiturate, barbiturate pill, bedevilment, black stuff,
   blank despondency, blight, blue, blue angel, blue devil,
   blue heaven, blue velvet, bore, bother, botheration,
   bothersomeness, bummer, calmative, care, chloral hydrate, codeine,
   codeine cough syrup, crashing bore, cross, curse, death wish,
   dejectedness, dejection, depressant, depression, depressor,
   despair, despondency, despondentness, devilment, difficulties,
   difficulty, discouragement, disheartenment, dispiritedness,
   dogging, dolly, down trip, downcastness, downheartedness, drag,
   drooping spirits, exasperation, goofball, harassment, hard knocks,
   hard life, hard lot, hard stuff, hardcase, hardship, harrying,
   headache, heartlessness, heroin, hop, hopelessness, horse,
   hounding, hypnotic, irritation, junk, knockout drops, laudanum,
   liquor, lotus, low spirits, lowness, lowness of spirit, malaise,
   meperidine, methadone, molestation, morphia, morphine, narcotic,
   nuisance, opiate, opium, oppression, pacifier, pain killer,
   paregoric, pen yan, persecution, pessimism, pest, phenobarbital,
   phenobarbital sodium, plight, predicament, pressure, problem,
   purple heart, quietener, rainbow, red, rigor, scag,
   sea of troubles, secobarbital sodium, sedative,
   self-destructive urge, shit, sinking heart, sleep-inducer, sleeper,
   sleeping draught, sleeping pill, smack, sodium thiopental,
   somnifacient, soother, soothing syrup, soporific, spiritlessness,
   stress, stress of life, suicidal despair, taedium vitae, tar,
   tranquilizer, trial, tribulation, trouble, troubles, turps,
   vale of tears, vexation, vexatiousness, vicissitude,
   weariness of life, white stuff, worriment, worry, yellow,
   yellow jacket