The US Food and Drug Administration (FDA) said it was concerned that 32 psychiatric events, such as hallucinations and abnormal behaviour, had also been reported in children who took Tamiflu, all but one of them in Japan.

“Any time you get a report of a death or a serious occurrence, you want to look into it,” Dr Murray Lumpkin, the deputy commissioner of the FDA, said.

The deaths of 12 children, aged 1 to 9, included one suicide, four cases of sudden death and four cases of cardiac arrest, as well as single cases of pneumonia, asphyxiation and acute pancreatitis. They all took place since 2000.

The FDA said: “The level of detail in these reports was highly variable, and determining the contribution of Tamiflu to the deaths was difficult.”

However, the FDA’s advisory panel of independent scientists later said that no harmful effect of Tamiflu had so far been discovered. “If we ever have a pandemic of avian flu, which is a debatable point, people want to know that they have a drug that will not cause more (harm) than the flu itself,” Dr Robert Nelson, chairman of the FDA’s Paediatric Advisory Committee, said. “There is no evidence that it will.”

Millions of doses of Tamiflu are being stockpiled around the world as bird flu spreads from Asia, where it has killed at least 64 people. Experts fear a worldwide pandemic if the virus mutates into a form that can be passed easily between people. Japan, where Tamiflu is routinely used during the winter flu season, is the only country with extensive experience of the drug. Of 32 million people treated with Tamiflu since its approval in 1999, 24 million were in Japan.

The Japanese Health Ministry issued a warning last week that Tamiflu might induce strange behaviour after the deaths of two teenage boys shortly after taking the drug.

Roche, the Swiss manufacturer of the drug, said that it had “carefully reviewed these events and has concluded that a causal link cannot be established”. Roche, which plans to increase production of Tamiflu tenfold by the end of next year, said that several studies had shown that the incidence of death in influenza patients who took Tamiflu was far lower than in those who did not.

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