Synonyms for Toxicant (same or very similar meaning)
WordNet sense 1 (any substance that causes injury or illness or death of a living organism):
poison, poisonous substanceWordNet sense 2 (having the qualities or effects of a poison):
poisonousWordNet sense 3 (of or relating to or caused by a toxin or poison):
toxicFrom the ODE community, based on WordNetadd/edit