The US Food and Drug Administration has raised concerns about how safe it is for children to take Tamiflu.
Tamiflu is seen as a key weapon in the event of a pandemic
In a memorandum, the FDA says it is aware of 12 deaths of children who took the drug in the past 13 months.
Tamiflu is the most important anti-viral drug in the fight against bird flu in humans.
Manufacturer Roche says the rate of deaths and psychiatric problems is no higher among those taking its medication than among those with flu.
The FDA memorandum mentions 75 cases that raise concern about the use of Tamiflu in children.
Among the 12 deaths, four are described as sudden, one attributed to a fall during a psychiatric disturbance, and several others to heart and lung failure.
There are also 32 cases of psychiatric disturbance, including hallucinations.
The report says there is not enough detail to assess the cause of these cases, but that the pattern is so unusual as to warrant further study.
Bird flu is unusual in having a serious effect on the young, and is more lethal to children than adults.
Roche is increasing Tamiflu production to 300 million doses a year to meet worldwide demand.