If held by a Medicham, allows it to Mega Evolve into Mega Medicham if its Trainer has a Key Stone. Each Trainer can only Mega Evolve one Pokémon per battle.
Unaffected by item negating effects (such as Embargo and Klutz). Item manipulating effects (such as Trick and Fling) cannot give Medicham a Medichamite or remove a Medichamite from Medicham.
Outside of battle
If held by a Medicham, allows it to Mega Evolve into Mega Medicham when it performs a Spectacular Talent during a Pokémon Contest Spectacular while its Trainer has a Key Stone. Medicham will remain a Mega Medicham until the Talent Round ends.