It would be pretty easy to tell a plausible lie, though, especially if you can think of a plausible less-painful poison to pretend you used. Tell them it's some

It would be pretty easy to tell a plausible lie, though, especially if you can think of a plausible less-painful poison to pretend you used. Tell them it's something moderately painful, so you can still tell them, "look, it's still better than torture". Just don't remind them about the edge-case scenarios where it really has to be fast and reliable, and they probably won't question it too hard...