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Poisons the target. Pokemon protected by Magic Coat or the ability Magic Bounce are unaffected and instead use this move themselves.
If the existence of Toxic, a move that 100% outclasses PoisonPowder, isn't enough reason to not run it, there are plenty more reasons besides. The fact that all Pokemon that learn it can also learn Stun Spore, Sleep Powder, and the powerful Spore also spells doom for PoisonPowder. The fact is that paralysis and, most importantly, sleep are much more important forms of support than both forms of poison. Don't use PoisonPowder, as all it learners always have a better status move, better even than Toxic.
Toxic is a better alternative, as it inflicts Toxic poison instead of regular poison.