Any link between the antiviral Tamiflu and neuropsychiatric problems is inconclusive, according to the Japanese Government.
The announcement comes after an investigation following earlier reports of abnormal behaviour and self harming in younger patients who had taken the Roche-manufactured flu drug.
Since 2005, there have been more than 100 reports of abnormal behaviour in people taking Tamiflu (oseltamivir), including eight deaths.
According to the Health Ministry report, almost 60 per cent of people showing abnormal behaviour had taken the drug.
However, figures were unclear as the number of influenza patients prescribed Tamiflu was unknown, the report said.
The group who wrote the report also found there was no reduction in the number of cases of abnormal behaviour following the government's recommendation in March that the use of Tamiflu in teenage patients be avoided.
A second separate study, which is hoped to be more conclusive, is expected to be presented before the year's end.
Last month, Roche's Japanese manufacturer and distributor of Tamiflu, Chugai, said Japan would only be receiving half of its usual supply of the drug following reduced demand because of the reports of abnormal behaviour and the government's warning about the use of the drug in teenagers. Japan will now receive six-million courses of the drug, not the 12-million that has been supplied in previous years.
While both Roche and Chugai have maintained that no causal relationship has been established between taking Tamiflu and the alleged psychiatric adverse events, the drug's prescribing information has been amended to include a warning pertaining to "incidences of self-injury and delirium", seizure and confusion.
The World Health Organisation recommends Tamiflu as the primary antiviral for patients infected with H5N1 influenza.
Between 2004 and 2006, Roche increased manufacturing capacity by 15 per cent, but earlier this year the company completed its pandemic stockpiling orders and has since scaled back manufacturing.
The company, however, has the capacity to produce more than 400 million treatments annually from eight Roche manufacturing sites and 19 external manufacturing partners, which are located in nine countries around the world.