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Raises Ditto's Defense and Special Defense by 50%. If Ditto transforms into another Pokemon, the boosts are lost.
Metal Powder doubles Ditto's Defense and Special Defense, which is useful as it has awful stats and needs time to use Transform. Unfortunately, the boosts are lost when Ditto transforms, meaning that once you have successfully transformed, you have effectively no item. However, Ditto itself is not a very good or useful Pokemon. Also note that once Ditto receives its Dream World ability, Imposter, it will almost always run Choice Scarf over Metal Powder, as Imposter enables it to Transform into the foe as soon as it is brought into the field.
Wild Ditto (5%).