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Focus Energy actually reduces the user's critical hit rate. If the attack deals critical hits sometimes, then the chance of its happening is quartered.
If a move has a high chance of dealing a critical hit, such as Razor Leaf, then its reduction depends on the current Speed of the user. If it is currently faster than the opposing Pokémon, then its critical hit ratio is not decreased. If the user is somewhat slower than the opposing Pokémon, then its chances of dealing a critical hit is cut by 50%. If the user is significantly slower than the opposing Pokémon, then the user will be unable to deal critical hits to the opposing Pokémon. In Pokémon Stadium, this glitch does not apply.
Never use this move.