Dr. Boyle is the man who sells Homer the Dimoxinil hair growth medicine. When Homer first visits the Hair Clinic, he is devastated to learn that Dimoxinil costs $1000, and has to turn Boyle down because he can't afford it. Boyle then attempts to sell Homer "Hair In A Drum", a much cheaper and much less effective alternative, which Homer rejects.
After Lenny gives Homer the idea to charge the Dimoxinil to the Springfield Nuclear Power Plant's health insurance plan, he returns to the Hair Clinic. Dr. Boyle peruses the form, yells at Homer that this is an obvious case of insurance fraud and declines the sale. He then turns on a radio and whispers to Homer to go outside in fifteen minutes, where he clandestinely gives the Dimoxinil to Homer in a back alley.