Brainiac.

Brain chemistry research has produced a drug - Nirvana - that can wipe out addiction and mental illness by stimulating pleasure centers in the brain and flooding the brain with endorphins. PsyMax Labs is about to release Nirvana, but suddenly something goes badly wrong. Nirvana causes drastic genetic changes, switching on dormant genes to produce BRAINIAC, a creature whose need for the brain's pleasure chemicals causes it to begin eating human brains.