TOKYO, April 5 (Reuters) - Shares of Chugai Pharmaceutical Co. Ltd <4519.T> weakened on Thursday after more than 100 people were found to have demonstrated abnormal behaviour after taking the influenza drug Tamiflu.

Among 1,079 cases of patients experiencing side effects after taking Tamiflu that have been reported by Chugai, 128 of them, mostly young people, were classified by Japan's Health Ministry as having shown abnormal behaviour.

The time period covers from early 2001, when Chugai -- the Japanese partner of Tamiflu manufacturer Roche Holding AG <ROG.VX> -- began selling the drug until March 20 this year.

Fresh cases of teens jumping from buildings after taking Tamiflu prompted Japan's Health Ministry last month to warn against prescribing the drug to those aged 10 to 19 and say it would launch a full investigation into whether there was a causal link between the drug and psychiatric problems.

It had previously maintained that there was no link.

Of the 128 cases, 57 were teenagers while 43 were under 10 years old.

There have been eight cases of deaths after abnormal or possibly abnormal behaviour. Sixty-seven people have died after taking the drug.

Tamiflu has been widely prescribed in Japan to treat influenza, with Chugai estimating some 35 million people have taken the drug, accounting for around 70 percent of the world's Tamiflu consumption.

Tamiflu is considered one of the best defences against bird flu, with governments around the world stockpiling the drug. Chugai's stock was down 0.9 percent at 2,890 yen in afternoon trade. Shares of Toyama Chemical Co. Ltd <4518.T>, which has a drug to combat bird flu in phase I development, jumped 5.2 percent to 790 yen.

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