The Collaborative International Dictionary of English v.0.48:

Toxicant \Tox"i*cant\, n.
   A poisonous agent or drug, as opium; an intoxicant.
   [1913 Webster]

WordNet (r) 3.0 (2006):

toxicant
    adj 1: having the qualities or effects of a poison [syn:
           poisonous, toxicant]
    n 1: any substance that causes injury or illness or death of a
         living organism [syn: poison, toxicant, poisonous
         substance]