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Faints the user. Damage is calculated as though the defender has half its Defense.
If the opposing Pokémon survives, it will not be charged with 1 PP for the attack selected if it was second to go. It will also be unaffected by poison, burn, and Leech Seed. If the opponent's Pokémon has a Substitute up, then a Pokémon who uses Selfdestruct will not faint. However, if Selfdestruct is used on a Substitute while playing Pokémon Stadium, then the user will faint.
An excellent last resort move and a good way to deal with Amnesia users. Despite not being as powerful as in the following generations, it's a better move in RBY than it is in BW, granting an extra kill most of the time for Pokemon that are about to faint anyways. The best user of this move is undoubtedly Snorlax, who gets STAB to cover for the lack of Base Power. It's a way to deal with Chansey, but keep in mind that you will need to weaken opposing Snorlax to KO it with this move, as it has a decent Defense stat. In Ubers, it's a good way to get rid of Mewtwo before it stacks a second Amnesia or sweeps you with Psychic.
Explosion is a stronger version of Selfdestruct that is also Normal-type.