Researchers have analysed a group of 4,000 Potter fans and found that ten per cent of them suffer from symptoms like depression, loss of appetite and sleeplessness, while 20 per cent are on the verge of addiction.

Till date, a total of 400,000,000 copies of the seven book series, including Harry Potter And The Deathly Hallows published last summer, have been sold worldwide.

"An addiction to a drug is no different from an addiction to Harry Potter or the Internet or pornography. It's not always a bad thing. There's a community that you get with Harry Potter that you don't get with heroin.

"Some readers can become so engaged in the series and the ancillary world that grew out of it that they report behaviours -- that truly fit definitions of addiction or dependence," the Daily Mail quoted lead researcher Professor Jeffrey Rudski of Muhlenberg College, Pennsylvania, as saying.

In their study, the researchers also compared cravings for cigarettes to that for the book.

"After finishing the series, ten per cent of fans spent over four hours a day on Potter-related activities, experiencing interference with appetite and sleep, engaging in less physical activity, having a lower sense of well-being and being more irritable," according to the researchers.