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This Pokemon's Attack and Speed are reduced by 50% for 5 turns after it switches in. If the user switches out, the counter resets. This ability can be removed by Skill Swap, Gastro Acid, or Worry Seed. If Slow Start is skill swapped onto a Pokémon already under the effects of Slow Start, the counter is not reset. Slow Start has no effect outside of battle.
In Japanese versions of Diamond / Pearl, removing Slow Start does not undo the Attack and Speed drop; the Slow Start Pokemon must still wait 5 turns.