The Collaborative International Dictionary of English v.0.48:

Toxicologist \Tox`i*col"o*gist\, n.
   One versed in toxicology; the writer of a treatise on
   poisons.
   [1913 Webster]

WordNet (r) 3.0 (2006):

toxicologist
    n 1: one who studies the nature and effects of poisons and their
         treatment