Ring of Pain

Regular health potions are a light pink color. Curse potions are purple with 3 little ghosts on them. When you select one, you'll see the curse percentage in the lower left of the screen. If you lose health from drinking it, you successfully cursed yourself. There's no real need to sweat this cheevo because cursed potions are relatively common, especially after the first few floors. You can increase your chances of becoming cursed by selecting items with negative curse resistance, such as the Fractal gem. The Masochist mask can help you with this cheevo because it heals you when you drink a cursed potion, increasing the chances that you will survive drinking several of them consecutively.