Go with something mild, then. Have them get drunk on alcohol, without drinking water during that period, and see if they wake up with a headache. If they do it obviously didn't work, as alcohol causes dehydration, which is harming them. Come to think of it, them getting drunk is also because of the harm of alcohol, so if they can get drunk it obviously doesn't work.
Oh, and they can't say it's not deadly, either, as people can die from alcohol poisoning (and they don't need to get anywhere near that amount to have the point proven or disproven).