Great thing about medicine is you can do as much as your motivation allows. Tox is the obvious choice, but any generalist like emergency or family medicine will be dealing with this as well. If you do end up in toxicology the vast majority of your cases will be: baby got into something, somebody tried to OD on purpose, somebody ODed accidentally either because they're old or because they were trying to get high. Trying to make a practice 100% out of envenomation is impractical and you would be better off studying that specifically in undergrad/grad school and going the phd route, but if you want to be a doc and envenomation is interesting to you, do the legwork to find out as much as you can about it, become the expert, give talks and such, and the work will generally find you.