To poison someone in Hitman you need to first find some poison on the mission you are playing. This could be in a vial, or as is most common, rat poison. To poison someone, you need to administer the poisonous substance to something that the target will then eat or drink. An easy place to unlock this achievement is during the mission 'Bjarkhov Bomb' by applying the laxatives to the soup that Fabian Fuchs subsequently eats. The laxative bottle is in the kitchen of the mess hall, you should apply it to the pot cooking on the stove when no-one is looking. As soon as he eats the soup, the achievement will unlock.