Shaggy & Scooby-Doo Get a Clue! (2006)

S2E10 Don't Get a Big Head

Frustrated by being outwitted by the gang, Dr. Phibes enhances his nano formula to boost his IQ, but it also causes his head to swell. Now, it's up to Shaggy and Scooby to prevent him from unleashing a gas that would significantly diminish the intelligence of people everywhere.