The Collaborative International Dictionary of English v.0.48:

Poisonous \Poi"son*ous\, a.
   Having the qualities or effects of poison; venomous; baneful;
   corrupting; noxious. --Shak. -- Poi"son*ous*ly, adv. --
   Poi"son*ous*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

poisonous
    adj 1: having the qualities or effects of a poison [syn:
           poisonous, toxicant]
    2: not safe to eat
    3: marked by deep ill will; deliberately harmful; "poisonous
       hate"; "venomous criticism"; "vicious gossip" [syn:
       poisonous, venomous, vicious]

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "poisonous":
   bad, baleful, baneful, barfy, brackish, catching, cloying,
   communicable, contagious, corroding, corrosive, corrupting,
   corruptive, counterproductive, damaging, dangerous, deadly,
   defamatory, deleterious, destructive, detrimental, diabolical,
   disadvantageous, disserviceable, distressing, envenomed, evil,
   fatal, fetid, foul, fulsome, harmful, high, hurtful, icky,
   infectious, infective, injurious, lethal, maggoty, malefic,
   malevolent, malicious, malign, malignant, mawkish, mephitic,
   miasmal, miasmatic, miasmic, mischievous, mortal, nasty, nauseant,
   nauseating, nauseous, noisome, noxious, offensive, ominous,
   overripe, pernicious, pestiferous, pestilent, pestilential, poison,
   prejudicial, rancid, rank, rebarbative, rotten, scatheful, septic,
   sickening, slanderous, spoiled, stinking, toxic, toxicant,
   toxiferous, venenate, veneniferous, venenous, venomous, vicious,
   vile, virulent, vomity, weevily, yucky