From this page of the Hitman 3 game guide, you will learn what types of poison are in the game, what effects they have, and how to poison your opponent.
Poisoning a meal or drink is one of the preferred methods to eliminate a target or induce regurgitation. Playing Hitman 3, you'll find two types of poison: Emetic Rat Poison, which causes the target to vomit, and Lethal Poison, which instantly eliminates the target. Once you have gained the appropriate level of mastery, you can also unlock other types of poison that you can smuggle with you on the mission.
Emetic Rat Poison
Emetic Rat Poison is one type of poison you can find in Hitman 3. It is usually stored in utility rooms, storage rooms, bathrooms, or other service rooms. You will find at least 3 doses of this substance on most maps.
Emetic Rat Poison is not lethal, but it does make the target nauseous, causing him to go to the nearest toilet. Thanks to this, you gain the opportunity to eliminate the target and/or to obtain an appropriate disguise.
Lethal poison comes most often in pill form (Lethal Poison Pill Jar). Unlike rat poison, it kills the target immediately. Lethal Poison is much rarer and harder to get than Rat Poison, and you will usually only find one dose of this substance on the map.
A victim who tastes poisoned food or drink begins to choke immediately and then falls to the floor and dies on the spot. Poisoning the target does not expose Agent 47, keeping him outside the circle of suspects.
How to poison your opponent?
One way to poison an enemy is to prepare a meal for them and add Emetic Rat Poison or Lethal Poison. Poisoned food will emit a distinctive colored smoke: green for rat poison and pink for lethal poison.
The second way to poison your opponent is to add the appropriate poison to a water bottle or drink glass. As with poisoned food, the bottle or glass will begin to smoke characteristically.
A third way to poison an enemy is to add a poisonous substance to the ventilation system. If this is the case, make sure the target is in the correct room and that the air conditioning is on.