Dr Howard Bennett said three children complained they were suffering from headaches after reading the 870 pages of the latest Potter book.
The doctor from the George Washington University Medical Centre calls them "Hogwarts headaches", after the school that Harry goes to.
Two of the kids didn't want the cure - taking a break from reading the books.
Dr Bennett told a US medical journal that the three children - all aged between eight and 10 - had bad headaches for several days.
In each case, the headache went away only after the patient turned the final page.
And with each of the five books so far longer than the one before, the doctor is worried even more kids are going to get "Hogwarts headaches".
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