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):

tranquillizer
    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]