WordNet sense 1 (a drug used to reduce stress or tension without reducing mental clarity):

tranquilizer, tranquillizer, tranquilliser, ataractic agent, ataractic drug, antianxiety agent

WordNet sense 2 (tending to soothe or tranquilize):

sedative, ataraxic, tranquilising, tranquilizing

WordNet sense 3 (capable of depressing physiological or psychological activity or response by a chemical agent):

depressant