Potions can be used for duelling. You could use dark potions to harm others or someone could slip dark potions into your drink. To avoid others spoiling your drink with dark potions, do not leave it unattended. Also, it is important to have good knowledge of Dark Potions so that you can recognize their effects and seek antidotes or help.

The Confusing Concoction- It harms your opponent’s ability to think, as well as causes the indirect effect that they are in harm’s way without someone to care for them. There is also no known antidote to this potion, since that would require an in-depth understanding of how the mind works, something that scientists - both magical and non-magical - are still working on. All that can be done is wait for the effects to wear off.

This applies to all potions that have effects that cannot be countered, but that are not permanent. When the effect is not permanent, but also non-lethal, the person who applied the potion has a moral and ethical responsibility to care for the drinker(except in a combat situation). - Defense Against the Dark Arts Lesson 7, Seventh Year, Hogwarts School of Witchcraft and Wizardry.

Draught of Living Death- When brewed properly, it places the drinker in a deep, comatose-like slumber from which they are unable to awaken naturally. The effects, when the potion is brewed improperly, depends on where the brewing process went wrong. For example, if the addition of the Valerian Root is done incorrectly, the potion may become toxic and cause the person not only to fall asleep, but to slowly die in their sleep. - Defense Against the Dark Arts Lesson 7, Seventh Year, Hogwarts School of Witchcraft and Wizardry.

Weedosoros- a poison so potent that even the smallest drop will cause you severe harm. Early symptoms of consumption include convulsion, but as the poison continues to progress through your system, it will begin to slowly shut down the organs that it comes into contact with until, eventually, all of your bodily functions will have been shut down. The process will be painful. It is not dangerous if it gets on your skin - it will not leave any burn marks - but you should immediately wash the area where the potion hits if you are unfortunate to be hit by splash. If ingested, the ability to cure it depends entirely on the dosage. Even at the dose of a few drops, it takes an exceptionally powerful antidote to cure; use the average dose of one mL of this potion and no antidote will cure them. If, by some accident, you consume Weedosoros, you should immediately seek medical assistance. - Defense Against the Dark Arts Lesson 7, Seventh Year, Hogwarts School of Witchcraft and Wizardry.

The Emerald Potion- It induces fear, delirium, intense stomach pain and extreme thirst in the drinker. It is emerald green in colour and glows phosphorescently. It was used by the notorious wizard Voldemort to protect his Horcrux, the Slytherin's locket.