Following reports of delirium, hallucinations and suicidal behavior among young people in Japan who took the flu drug Tamiflu, the US FDA and the drug's maker have added a warning to its label.

The agency is advising that doctors and users of the drug be alert to the possibility of delirium, hallucinations or other abnormal effects.

More than 100 instances of strange psychiatric reactions, including suicidal behavior, have been reported among Tamiflu users in Japan -- usually within a day of taking the drug. The FDA has reviewed those cases.

The majority involved people under 17 years of age, three of whom died from falls. Some experts have questions whether the abnormal behavior reported was the result of taking the drug or a consequence of the flu, which can have neurological effects in rare cases.

Tamiflu is used to reduce the severity of influenza and is recommended particularly for patients at high risk of severe side-effects. It is one of just a few drugs that is considered a potentially effective treatment for avian influenza, or H5N1 bird flu.

Bird flu, which has claimed 150 lives since it emerged in 2003 is being closely monitored. Experts fear it could mutate into a form of flu easily transmitted from one person to another, triggering a global pandemic that could kill millions.