If you use the Grepa Berry on a Pokemon with less than 10 special defense EVs, it will decrease the special defense EVs by the number of special defense EVs you have. For example, if you have a Pokemon who has 8 special defense EVs and you have it consume a Grepa Berry, it will decrease the amount of special defense EVs by 8.
The amount of items that a Pokémon can consume is proportional to the number of EVs it has in its respective stat. For example, a Pokémon that can eat two Pomeg Berries before its HP stops decreasing has between 11 and 20 EVs in HP.
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Here's another example using the thing about eating two friendship raising berries. If you had a Pokemon with 16 special defense EVs, it can eat one Grepa Berry to decrease the special defense EVs by 6.
This Berry increases a Pokémon's friendship by and lowers its Special Defense EVs by ten points (but not below 0)
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Still using the 16 special defense EVs example that's down to 6 special defense EVs, because the EVs will not drop below 0, it will decrease by the amount of special defense EVs you have, meaning a Pokemon with 6 Special Defense EVs that eats a Grepa Berry will have its special defense EVs decreased by 6.
Hope this helps.
