The health ministry has reported that 151 flu patients up to age 17 demonstrated abnormal behavior between late September and mid-November, including acting violently or uttering gibberish.
Most of the cases are believed to have involved patients with swine flu, and the strange behavior is thought to have occurred regardless of whether they were given the drug Tamiflu, according to a report submitted to a research group at the Health, Labor and Welfare Ministry.
The number of cases, coming in a period of less than two months, is already close to the total of 179 seen in all of last year's flu season.
A ministry official said cases this year have been on the rise in line with the spread of the new H1N1 flu and young patients should be watched carefully for at least two days after developing a fever.
Of the 151 patients, it is known that Tamiflu was used in 26 cases, Relenza in 36 and neither of them in 16.
Tamiflu has been banned for use among teenagers out of fear it may cause abnormal behavior, though it can still be used in some cases.