I was also thinking of Nembutal, but because of the amount needed it would have to be a very large pill! :D
However, humanity will very likely find a new substance in the future, not known yet, which will not be required to be taken in larger amounts, and therefore would fit in one pill. When I think about it, I am even willing to believe the intelligence agencies around the world might already have a classified pill of this kind to be given to agents for the most important or dangerous missions.