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Deals damage and has a 50% chance to lower the target's Defense by one stage.
On paper this seems like a good move, sporting usable Base Power and a 50% chance to lower the opponent's Defense, which is great news for a physical move. However, competitively speaking it's outclassed by Return or Frustration as a Normal-type move. Both of these moves deal more damage on the first turn, which can be crucial, and even if the target's Defense is lowered after the first attack, both moves still deal more damage than Crush Claw in two turns regardless. Finally, Crush Claw has a 5% chance to not hit and Return/Frustration are still 100% accuracy moves, so favor the prudent battlers who do not want to miss at a critical moment.
Return has a higher Base Power and perfect accuracy, making it a much more reliable option.
Double-Edge is a much more powerful alternative; however, it inflicts recoil damage upon the user.