The comments come in response to weekend news reports that Japan's health ministry is investigating the deaths of two teenage boys who died in accidents linked to odd behaviour shortly after taking the drug.

Health ministry officials were not available for comment.

Shares in Chugai were down 3.1 percent at 2,630 yen on Monday afternoon, compared with a 1.3 percent fall in the Tokyo Stock Exchange's pharmaceutical sector subindex.

Tamiflu, considered one of the best defences against bird flu in humans, might help slow the spread of a much-feared pandemic should the H5N1 flu virus become able to spread from person to person.

The Mainichi newspaper and Kyodo News agency reported on Saturday that a 17-year-old high school student jumped in front of a truck in February last year shortly after taking the medicine, while a junior high school student is believed to have fallen from the ninth floor of his apartment building this February.

"We reported these cases to the health ministry as a link between the deaths and the drug could not be ruled out," a Chugai spokesman said. The reports were made separately after each incident, he said.

He said Chugai has included in the literature accompanying the drug a list of side effects such as impaired consciousness, abnormal behaviour and hallucinations and has called doctors' attention to the possible side effects.

Kyodo said the ministry is considering issuing a fresh warning about the side effects, following its decision to increase stockpiles of the drug amid growing fears about a possible pandemic.

The Pharmaceuticals and Medical Devices Agency said there were 64 cases of psychological disorders linked to the drug between fiscal 2000 and 2004, according to Kyodo.

Chugai launched Tamiflu in Japan in 2001. During the last flu season it shipped the drug to more than 10 million people.

The Japanese government is planning to boost its target stockpile of Tamiflu to 250 million capsules, up 70 percent from its previous target, to cover treatment for 25 million people.