WordNet (r) 3.0 (2006):

tranquillising
    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]