The Collaborative International Dictionary of English v.0.48:

ataraxic \ataraxic\ n.
   1. drug that reduces nervous tension and gives peace of mind.

   Syn: ataractic
        [WordNet 1.5] Ataunt

The Collaborative International Dictionary of English v.0.48:

ataractic \ataractic\ ataraxic \ataraxic\adj.
   1. tending to soothe or calm or tranquilize

   Syn: calming, sedative, soothing, tranquilizing
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

ataraxic
    adj 1: tending to soothe or tranquilize; "valium has a
           tranquilizing effect"; "took a hot drink with sedative
           properties before going to bed" [syn: ataractic,
           ataraxic, sedative, tranquilizing,
           tranquillizing, tranquilising, tranquillising]