I am considering making natural (nonmagical) herbs and poisons work identically, in the sense that both are chemicals introduced into a body that the body may or may not affect the individual. This would have a big effect, with respect to some races, especially halflings, dwarves, and elves - they would be very resistant to both poisons and herbal effects. I don't recall there being any rules along these lines, and wonder whether (a) I'm mistaken and there are rules, or (b) someone has done this and was happy with the results.
Magical herbs/poisons would be an altogether different issue, but I'd probably make them follow as similar a mechanic as possible.