Painless, compact, reliable? Doesn’t need to be that fast.
I thought the point was to remove you from enemy torture and questioning. If you're dying slowly, then the enemy is just going to torture and question you while the poison acts slowly, or possibly even find a way to ‘cure’ you well enough that the torture can go on for a long time … which defeats the point, doesn't it?
1) There is no antidote 2) It induces quick unconsciousness so you can't be tortured while it's working.
Maybe ricin?