The Collaborative International Dictionary of English v.0.48:

Toxic \Tox"ic\, Toxical \Tox"ic*al\, a. [L. toxicum poison,
   originally, a poison in which arrows were dipped, Gr.
   toxiko`n (sc. ?) poison for smearing arrows with, fr.
   toxiko`s of or for the bow, from to`xon bow, arrow. Cf.
   Intoxicate.]
   Of or pertaining to poison; poisonous; as, toxic medicines.
   [1913 Webster]

WordNet (r) 3.0 (2006):

toxic
    adj 1: of or relating to or caused by a toxin or poison;
           "suffering from exposure to toxic substances" [ant:
           atoxic, nontoxic]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "toxic":
   baleful, baneful, catching, communicable, contagious, corroding,
   corrosive, corrupting, corruptive, counterproductive, damaging,
   deadly, deleterious, destructive, detrimental, disadvantageous,
   disserviceable, distressing, envenomed, harmful, hurtful,
   infectious, infective, injurious, lethal, malefic, malevolent,
   malign, malignant, mephitic, miasmal, miasmatic, miasmic,
   mischievous, noisome, noxious, ominous, pernicious, pestiferous,
   pestilential, poison, poisonous, prejudicial, scatheful, toxicant,
   toxiferous, venenate, veneniferous, venenous, venomous, vicious,
   virulent