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Deals damage. The attack repeats itself for 2-3 turns, during which time the user is locked in play, after which the user becomes confused. The user only uses up PP on the turn the move is first used, and not on subsequent rampage turns. If the user is prevented from moving while rampaging, the attack misses, or is blocked by Protect or Wonder Guard, the attack will immediately end and the user can switch attacks. If this move is called by Sleep Talk, the move is used for one turn and does not confuse the user. The confusion side-effect will only occur if the user ends its Thrash naturally, and without disruption. The confusion is not preventable with Safeguard, but can be prevented by Own Tempo. In a double or triple battle, the target of this attack is randomly selected from all opposing Pokemon, even if they are not adjacent to the user.
Thrash should not be used competitively. Though the Base Power of the move is very high and sports perfect accuracy, Normal has a number of resistances and immunities, and does not hit any type super effectively. Locking the user into this move is rarely a good idea. Its distribution is also quite low.
Return is a less powerful but more reliable alternative.
Double-Edge has the same Base Power, is more reliable, and causes recoil damage.