For my upcoming sourcebook, I'm currently working on poisons. I've started with the 14 poisons from the SRD, updating them and rewriting them to conform to the general format I'm using for poisons in my book. Some of them are getting additional effects or information.
One thing I'm adding to some of them is a delay between when the poison is administered/ingested/whatever and when the poison takes effect (delaying the Con save as well so it's not immediately clear what caused the poisoning in some cases).
This particular poison helps get the truth out of someone, especially if you use a couple extra doses on them.
I added the spoken train of thought / spilling everything aspect thanks to inspiration from Luis in the 2nd Ant-Man movie.
Also, if you haven't seen it before or haven't in forever, I recommend True Lies, which has another fun "truth serum" scene. That's just a fun movie.