This is the preferred term for a substance that is considered to be toxic under circumstances which are thought likely to happen.
The term "toxicant" was found in the following pages:
Agilent e-Seminar: A Metallomics Approach for Analysis of Biological and Environmental Compounds via ICP-MS | EVISA's Agenda of Events
Link database: Toxipedia: Cadmium
ICCE 2009: 12th EuCheMS International Conference on Chemistry and the Environment | EVISA's Agenda of Events
Journals database: Reviews in Toxicology