LONDON , England ( CNN ) -- More than half of children taking antiviral drug Tamiflu suffer side-effects such as nausea , insomnia and nightmares , UK researchers have said . Anti-viral drug Tamiflu caused nausea , insomnia and nightmares among some children studies show . Two studies from experts at the Health Protection Agency ( HPA ) showed a `` high proportion '' of British schoolchildren reporting problems after taking the anti-viral drug . Data was collected from children at three schools in London and one in south west England who were given Tamiflu earlier this year after classmates became infected . The studies by HPA experts were carried out in the early stages of the epidemic between April and May , when everyone sharing a classroom with a child who developed swine flu was given the drug , even if they showed no symptoms . Their research , published in the Eurosurveillance scientific journal , looked at side-effects reported by 11 and 12-year-old pupils in one school year in a secondary school in south west England . A total of 248 pupils took part in the study ; their school was closed for 10 days after a pupil there contracted swine flu during a holiday in Mexico , researchers said . Seventy-seven percent of these children took the full course of Tamiflu , according to researchers . The report said : `` Fifty-one percent experienced symptoms such as feeling sick ( 31.2 percent ) , headaches ( 24.3 percent ) and stomach ache ( 21.1 percent ) . '' One of the studies found that evidence of side-effects prompted UK ministers to change the policy so only confirmed or suspected cases were prescribed Tamiflu . The second study , also published by Eurosurveillance , looked at 103 children . The research found that 53 percent of children at three London schools had one or more side effects . The most common side effect was nausea , with 29 percent of the pupils complaining of this . Another 20 percent of those questioned by researchers complained of stomach pain or cramps , while 12 percent had problems sleeping , the study found . Almost one in five ( 18 percent ) of the children reported a `` neuropsychiatric side effect , '' such as poor concentration , inability to think clearly , problems sleeping , feeling dazed or confused , bad dreams or nightmares and `` behaving strangely , '' researchers said . The UK 's Department of Health said in a statement : `` The European Union regulatory position remains that no causal association between Tamiflu ( or Relenza ) and an increase in neuropsychiatric events has been established . '' Reports from Japan , where Tamiflu has in the past been widely-used against seasonal flu , has linked the drug in rare instances with unusual neurological and psychiatric disturbances in children , according to media reports . Japan is now advising against prescribing Tamiflu to youngsters aged 10 to 19 after its own studies revealed people reporting psychiatric symptoms , according to media reports . A spokeswoman for pharmaceutical company Roche , which manufactures Tamiflu , told CNN : `` Abnormal behavior is recognized to occur in patients with influenza virus infection including those taking and not taking antiviral drugs . `` Following extensive investigations , no mechanism has been identified by which Tamiflu could contribute to such events . Following a review by an FDA advisory committee in the US , the labels for both Relenza ( zanamivir ) and Tamiflu have been updated to inform physicians that influenza can be associated with a variety of neurologic and behavioural symptoms . ''
Researchers say more than half of children who took Tamiflu suffered side-effects . Among reactions were nausea , nightmares and insomnia in . Japan is advising against prescribing Tamiflu to youngsters aged 10 to 19 .