Not all Poisons...
... Are Created Equal
I was recently reading I, Strahd: The War Against Azalin (inspirational reading while I run Curse of Strahd) and something mentioned in the book sparked an idea.
In the book, Strahd creates a potion he plans to give to one of his human servants. After the human ingests the potion, the servant collapses, and Strahd fears he may have made the potion too strong. Later, he attempts to give a similar potion to a captured vampire enemy. This potion was crafted to be 10 times as strong as the previous one, knowing that it needed to be so potent to affect the vampire.
This got me thinking about when PCs find vials of poisons in treasure hordes, which always seemed a little meta-gamey to me. Why would they be there, if not for PCs to find later? Why do I have a poison in my treasure looking like any other tasty little potion, if not for the "gotcha" to the player when their character drinks it.
One way to rationalize this is to have the poison vials be intentional. Maybe the Big Bad has a bunch of them scattered around his treasure horde, within easy reach. Just the kind of item a quick snatch and grab might yield. This especially makes sense if the poison is something the Big Bad is already immune to, and shows some of the Big Bad's cleverness, stashing poisons for the characters to grab and take with them outside of the Big Bad's lair -- like rat poison, but for adventures!
Another way of justifying these poisons would be for them to not actually be intended as poisons, but that they are potions that are just too potent for the PC (like the Strahd case mentioned above). The advantage here is that the potion would still detect as magical. DMs would need to decide what would happen if the PC has a taste versus drinking the full bottle. Additionally, creative DMs could also work in the extra-potent potion effect as part of the description. Perhaps a potent Charm potion could cause psychic damage, madness, or physical damage (blood leaking out of the eyes, ears, and nose seem particularly gruesome). This may be a case where the Big Bad (say, a dragon), needs the potion to be extra-potent. To him, it's a potion, but to the weaker PCs, it's a bad day!
Lastly, consider that maybe these potions are not intended to be extra potent, but could just be the "mistakes" of apprentice mages. In this scenario, you could have potions that are extra-potent, less potent than a proper formulation (like a healing potion that only does 1d6), or just not effective (like an invisibility potion that makes your skin and organs invisible, but not your internal skeleton. Some of these "mistakes" might be picked up on the cheap, and be useful in the hands of clever characters.
So next time you stock that treasure horde, remember that poisons and potions need not be created equal.