Acne (2000)

Franny and Zoe wake up one morning ready to go on a high school field trip. Their plans quickly change when Zoe drinks contaminated water and his head becomes a monstrous “zithead", soon turning Franny into one too. Soon the two of them set off on a bizarre journey, mutating other teenagers, encountering strange suburbanites, and developing a very unusual craving for chocolate. The army starts a statewide manhunt for the infected adolecents as they struggle to remain concious, try to figure out what’s happened to them, and draw nearer to a terrifying fate.