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Deals damage. If used on a sleeping target, its Base Power doubles, but the target wakes up.
Wake-Up Slap has almost no competitive use. Sleep is arguably the most valuable status in the game. Wake-Up Slap, however, gets rid of that status. Furthermore, the only Pokemon, Breloom, that can put the opponent to sleep and also use Wake-Up Slap has other, better moves to use such as Focus Punch. Even when used on a sleeping target, Wake-Up Slap has only 120 Base Power, making it outclassed by other Fighting-type attacks such as Close Combat, which has a consistent 120 Base Power already. Wake-Up Slap should never be used unless there are no better alternatives. Cinccino is the only decent user of Wake-Up Slap due to Technician and its lack of another way to break through some of the Steel-types in RU.
SmellingSalt has a similar effect and the same Base Power, but it doubles in power when the opponent is paralyzed rather than sleeping.