I think the normal drawback (I think its a 10% chance) of poisoning yourself isn't balance enough? However, it definitely requires craft (poison) in terms of skill overlap and I disagree that it should be a professioin. Professions include things like fishing, farming, and sailing. Craft includes things like crafting weapons, crafting armor, crafting bows, ...crafting poisons. If you can put crafting in front of it (because it is something you make) it ought to be a craft skill. You don't make fish, or farm animals and crops. And crafting water vessels is different than being able to operate them. Certainly, poisonmaking is close to alchemy and herbalism, however, as rules supplements have already identified, poisonmaking requires a separate ability. I would give synergy bonuses from craft(alchemy) and profession(herbalist) for their respective types of poisons (chemical or plant) and probably also give bonuses when attempting to purify poisons from particular poisonous things such as vermin, plants for their respective knowledge skill.