Swiss drug maker Roche Holding and its Japanese partner Chugai Pharmaceutical will conduct new basic research and clinical trials on influenza drug Tamiflu, which is under strong scrutiny in Japan for a possible link to abnormal behavior.
Earlier this year, Japan's Health Ministry issued warnings against prescribing Tamiflu to teenagers, prompted by fresh cases of young people jumping from buildings after taking the drug.
The ministry also said it would launch a full investigation into whether there was a causal link between the drug and psychiatric problems. It had previously maintained there was no link.
Roche and Chugai have said no causal relationship has been established between Tamiflu and neuropsychiatric symptoms, and doctors say influenza itself can cause abnormal behavior.
Tamiflu is widely prescribed in Japan, with Chugai estimating some 35 million people have taken the drug, accounting for around 70% of the world's Tamiflu consumption.
It is less often prescribed in other countries as it is often not covered by medical insurance, although the drug -- seen as one of the first lines of defense against an outbreak of bird flu -- is being heavily stockpiled by governments.
The time period covers from early 2001, when Chugai began selling the drug, until April 17 this year.
There have been eight cases of deaths after abnormal or possibly abnormal behavior. Seventy people have died after taking the drug. There have been 26 cases of people either jumping off or falling from buildings or other heights, 22 of whom were teens.
Chugai said its study would be conducted on 12 to 30 human volunteers and would be completed by the end of December. An interim report would be presented by the end of September.