One of the skills I gave to my version of the thief class is brew poison. If the thief is not rolling for skill selection, brew poison will be gained at eighth level. Here is the text from the class description:
Brew poison: given 1 day and 100 gp, a thief can brew one dose of save-or-die poison sufficient to threaten the life of a human-sized opponent. How larger or smaller creatures react to poison is by referee ruling. 5 in 6 chance to identify and know effects of examined poisons. Other recipes (such as for a paralytic poison) can be found, or can be synthesized based on reverse-engineering an identified poison.
Brewing poison is actually a thing that any class can attempt, assuming they have a recipe and can procure the ingredients. I would probably make the components cost double for a non-thief. What really makes the thief ability special though is that the thief can brew poisons with no chance of accidentally poisoning themselves. For non-thieves, I use the following rule:
Given a recipe, ingredients, and basic facilities, anyone can attempt to brew poison. There is a 1 in 6 chance of accidental poison exposure. In that case, a standard save versus poison is required to avoid the poison effect.
The other two aspects of the thief ability that are special are that the thief basically gets the recipe for save or die poison for free and also gains the identification/reverse-engineering ability.