Medicine cabinets are used to heal your health..sleeping pills are usually within the level itself (example in the first mission it is in the 2nd floor bathroom )

Medicine cabinets are there to heal you, and can only be accessed when you are below full health.

Using Instinct will let you see what objects can be interacted with, though some will only be possible when you are carrying another item.

so on the first level you can find sleeping pills in the upstairs bathroom. You can take these to the kitchen and use them with the pot of food the chef is cooking, and you can also use them on the coffee cup on the wine bar that the head guard sometimes sits at.

It seems that once you have found the drug or poison for a particular level you can use it on all possible poisoning targets, and not just one use per find of poison.

You gotta get sleeping pills and then walk over to food or drink that can be drugged and press triangle. For example to get the Drugged challenge done in a personal contract, walk upstairs to the bathroom, grab the pills off the sink walk back to the kitchen and when the chef isnt looking walk over to the pot of food and wait for him to taste it.

I tried walking to food and pressing triangle but nothing happens, i'm confused how to do this

@fullfloater
First go upstairs and get the sleeping pills, then go to the kitchen and use them on the pot of food in the kitchen. Then you have to wait until the chef comes round the kitchen and tastes the food - that is when he gets drugged, and then goes to stumble over to the chair in the corner of the room.

Using drugs to take out a target is not an instant takedown, but the timing can be used to make your escape or move elsewhere in the level to take out another target.