Children are getting migraine headaches by reading the latest Harry Potter book, a Washington doctor has said.
Harry Potter and the Order of the Phoenix is nearly 900 pages long
Dr Howard Bennett said three children have complained of headaches caused by the physical stress of reading the 870-page Order of the Phoenix.
It has nearly three times as many pages as the first Harry Potter book.
The George Washington University Medical Centre doctor called them "Hogwarts headaches", after the wizard school that the book's hero attends.
In a tongue-in-cheek letter written to the New England Journal of Medicine, Dr Bennett wrote: "The presumed diagnosis for each child was a tension headache brought on by the effort required to plough through an 870-page book.
"The obvious cure for this malady - that is, taking a break from reading - was rejected by two of the patients," he said.
He added that in all cases the pain went away one or two days after the patient had finished reading the book.
To support his diagnosis, Dr Bennett used a chart showing how the length and weight of each of the Harry Potter books had increased each time, from 300 pages for book one to 870 for book five.
"If this escalation continues as Rowling concludes the saga, there may be an epidemic of Hogwarts headaches in the years to come," he added.
Meanwhile, sales of the series of five Harry Potter books on audio have topped the one million sales barrier.
The unabridged CD and cassette recordings read by actor Stephen Fry (Jim Dale in the US) last for more than 80 hours.
Steven Edney, sales director of BBC Audiobooks, said: "Harry Potter has
broken all the rules in the audio category and the brand is now worth over £30m."