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Deals damage and has a critical hit rate one stage higher than normal. Assuming there are no other critical hit modifiers in play, this move has a 1/8 chance to get a critical hit instead of a 1/16 chance. Has a 10% chance of poisoning the target.
Poison Tail isn't the best choice of a move. First, it's a Poison-type move, and thus suffers from lackluster coverage. Second, it's entirely outclassed by Poison Jab and Gunk Shot, which aren't even good moves regardless. Finally, while critical hits happen more often, its low power makes this almost irrelevant. In a nutshell, opt always for either Poison Jab or Gunk Shot.
Cross Poison is a more powerful equivalent.
Poison Jab is a more powerful option with a normal critical hit rate.
Gunk Shot is a much more powerful option, but has lower accuracy and a normal critical hit rate.