toxicant

https://doi.org/10.1351/goldbook.16240
This is the preferred term for a substance that is considered to be toxic under circumstances which are thought likely to happen.
Note: This word comes from the Greek toxikos "of or for the bow" and was originally applied to the poison used to tip arrows. The term "poison" is nearly a synonym, but tends to be applied to substances that may be deliberately used for poisoning, such as pesticides, and often has overtones of criminal use.
Source:
PAC, 2007, 79, 1583. (Explanatory dictionary of key terms in toxicology (IUPAC Recommendations 2007)) on page 1630 [Terms] [Paper]