The Collaborative International Dictionary of English v.0.48:

Tranquilize \Tran"quil*ize\, Tranquillize \Tran"quil*lize\, v.
   t. [imp. & p. p. Tranquilizedor Tranquilliized; p. pr. &
   vb. n. Tranquilizingor Tranquillizing.] [Cf. F.
   tranquilliser.]
   To render tranquil; to allay when agitated; to compose; to
   make calm and peaceful; as, to tranquilize a state disturbed
   by factions or civil commotions; to tranquilize the mind.
   [1913 Webster]

   Syn: To quiet; compose; still; soothe; appease; calm; pacify.
        [1913 Webster] Tranquilizer

WordNet (r) 3.0 (2006):

tranquilize
    v 1: make calm or still; "quiet the dragons of worry and fear"
         [syn: calm, calm down, quiet, tranquilize,
         tranquillize, tranquillise, quieten, lull, still]
         [ant: agitate, charge, charge up, commove,
         excite, rouse, turn on]
    2: cause to be calm or quiet as by administering a sedative to;
       "The patient must be sedated before the operation" [syn:
       sedate, calm, tranquilize, tranquillize,
       tranquillise] [ant: arouse, brace, energise,
       energize, perk up, stimulate]

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "tranquilize":
   allay, appease, arrange, array, balm, becalm, calm, calm down,
   cease, compose, conciliate, cool, cool off, cradle, defuse, deploy,
   die down, dispose, dulcify, dwindle, ease, ebb, even out, form,
   gentle, grade, halt, harmonize, hierarchize, hush, lay, lay out,
   lay the dust, line up, lull, marshal, methodize, molder, mollify,
   normalize, order, organize, pacify, placate, pour balm into,
   pour balm on, propitiate, quell, quiesce, quiet, quieten, rank,
   regularize, regulate, rest, rock, rock to sleep, routinize, sedate,
   set up, settle, smooth, smooth down, smooth over, smoothen, soothe,
   stabilize, standardize, steady, still, stop, structure, subdue,
   subside, systematize, wane