Potion, uncommon
This concoction looks, smells, and tastes like a Potion of Healing or another beneficial potion. However, it is actually poison masked by illusion magic. Identify reveals its true nature.
If you drink this potion, you take 4d6 Poison damage and must succeed on a DC 13 Constitution saving throw or have the Poisoned condition for 1 hour.
Notes: Damage: Poison
This will do really annoying things just by existing if you introduce it, both as a DM and a player.
1. Introducing its existence means the players, if they identify it properly, will distrust every single healing potion or just potion in general that they get from here on out.
2. People love hoarding health potions. Whether playing in-person or through something like a VTT or D&D Beyond, you'll need to pretend this is a health potion on their sheet or it'll be extremely obvious when the VTT/D&D Beyond says they have a "Potion of Poison" they don't remember collecting. Are you certain that in 50 sessions that you will remember it's even a potion of poison, and how you determine which one it is if you give them real potions of healings alongside it. In the best case scenario, you somehow remember to track which potion they've used out of all the different healing potion types they have, and they drink a poison they didn't properly identify thinking it was a health potion and get poisoned. At low levels, it might actually kill someone, which means the player will be super mad that you even bothered to give them a random evil poison potion that hurts instead of harms them, and at higher levels it does almost nothing since the damage is low and it's a con save.
I actually think the best use-case for this potion is for players to get a potion of poison for themselves intentionally and use it to trick an enemy. At least that might actually be cool. But, if you're trying to do that, why not use the plethora of awesome DMG poisons instead of this baby's first poison that, at maximum, does 24 damage and poisons an enemy for an hour and nothing else.
Overall, as a poisoner, this gets a 2/10 from me.