Rat poison worked in Dumb & Dumber -
On a serious note, rat poison could work, and I imagine it being very painful. I remember that guy who busted out a Cyanide capsule (or a few capsules) and took it when he realized he'd be going to prison.
I remember it working very quickly, and he was pretty stealth about it - when he put his hands to his face after the verdict (guilty of arson) was read, he slipped the pills into his mouth and not very long after he begins convulsing, I remember the noises he was making; between screaming and snarling - total loss of control over his body.