A change marked bottle (of vodka?)

Hello
I’m can’t even remember for the life of me if this is a movie or a TV series episode.
The only thing that I remember is that somebody, for some reason, has to kill somebody. For that, another character (I think some sort of spy or agent of government) provides him with some poison he has to administer to the target. He chose to do it pouring the poison in a bottle of white liquor (vodka, I think) which I he marked with an x. The whole thing happens in a hotel and when he returned to get the poisoned bottle he discovered that it has been taken to the meeting. At first nothing happens, he calms down and then everybody gets sick.
HELP!!!! My memory is mocking me