Just pretend you have a needle handy. It's a gameplay abstraction. Like, you know, hit points are. And being able to carry 500 pounds of stuff without so much as a backpack.
But it's abstraction that has no real world equivalent. Hitpoints stand in for blood loss and organ trauma. And by most people's guess, the weight units are equal to 1/4 of a pound, meaning that carrying 500 is actually 125 pounds, and that's only for Strength 100 chars. Of course, 125lbs is still a lot without physical representation, but the reason for that is aesthetics.
But if you want to poison someone the traditional, stealthy way, you put it in their food. Even in Oblivion this would have been possible as NPCs had eating schedules, although I they usually just pulled food from their inventory, only sometimes from the environment)
But now that we have cooking, it seems like it would be easier than ever to just click on someone's pan of salmon and drop the poison in there.
I just don't think putting a bottle of poison on someones pocket, or even spilling it all over them seems like a very good strategy. Either you have to assume that all people are dumb enough to ingest random liquids they find on themselves, or that the poison is so toxic as to work through the skin. But if it's really that deadly, you should have a pretty good chance of killing yourself. Unless you've built up an immunity. While that would be a cool feature in and of itself, it's another thing that should not have to be assumed in order to suspend disbelief.