A Mental Herb is an item introduced in Generation III that prevents the user from becoming infatuated with the opponent's Pokémon. This item can only be found, and can be sold for 
50.
In the Hoenn region games, the Mental Herb can be found in Fortree City and the Battle Frontier exclusive to Pokémon Emerald. In the Sinnoh region games, it can be found in Route 216 while in Pokémon HeartGold and SoulSilver, this item can be found in Route 7. In the Unova region games, this item can be found by wild Sewaddle, Leavanny, or a Swadloon.
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