The Antidote is a potion used to heal from poison, which is caused from attacks by some large insects and spiders, Noxious Gels, Venomous Wyverns, Venom Dragons, and other poisonous enemies. It stops the green bar from bringing your health lower and lower and changes the color of your health bar back to red. It can be fed to your pet for the same effect. It is about the size of a minor health potion,and is purple. It can be bought for 50 gp but it is usually more efficient to pick one up in the dungeon where they are very common.
It is very important to pick up from level 22 down, as Diseased Centipedes, Zombie Kings, Bugbear Necromancers, certain traps, Vile Tarantulas, Widow Queens, Venom Drakes/Dragons, and Widowlings all have extremely hurtful long-term poisons.