The Mutant Cure is an antibody developed by Worthington Labs at the former Alcatraz Prison. It makes it's only appearance in X-Men 3: The Last Stand.
X-Men 3: The Last Stand
Worthington Labs created the cure from the DNA of a mutant named Leech. This cure was to be administered to all mutants, whether willingly or not. The only problem was that mutants who received it, regained their powers slowly and that the cure merely suppressed their powers. This was clearly seen after Magneto was injected with the cure and he saw that one of the metal chess pieces he had slowly moved.