United States and European regulators have opened an investigation into the drug Tamiflu after the deaths of 12 Japanese children who took the medication.
But both US and European officials warned that it was difficult to tell whether the drug manufactured by the Swiss pharmaceutical giant Roche played a role in the cases.
The US Food and Drug Administration (FDA) said the 12 deaths it was reviewing included one suicide, four cases of sudden death and four cases of cardiac arrest. There also were single cases of pneumonia, asphyxiation and acute pancreatitis.
"The level of detail in these reports was highly variable and determining the contribution of Tamiflu to the deaths was difficult," the FDA said.
"At this point in time, we cannot make an association between Tamiflu and the deaths of these children," said FDA official Murray Lumpkin.
The FDA added that it was "concerned" that 32 psychiatric events, such as hallucinations and abnormal behavior, had also been reported in children who took Tamiflu. All but one case were reported in Japan.
The "most alarming" psychiatric events, according to the FDA, included two cases in which a 12-year-old and a 13-year-old jumped out of the second-floor windows of their homes after receiving two doses of Tamiflu.
The agency will ask for input on the cases from an advisory panel of outside experts at a public meeting on Friday.
The FDA's European counterpart, the European Medicines Agency (EMEA), said it was also studying the possible side effects of Tamiflu.
There have been two cases of "presumed suicide associated with the medication", the EMEA said.
The drug is in high demand because it is considered to be one of the best defenses against avian flu in people.
Interest in Tamiflu has risen as experts around the world warn of the possibility of an H5N1 bird flu pandemic among humans. Several countries are stockpiling Tamiflu, which may be able to lessen symptoms.
This week Roche reported two possible suicides of Japanese teenagers who took Tamiflu but said there was no clear evidence the drug contributed.
A company spokesman told the AFP news agency on Friday that the FDA evaluation was "routine" and would in total consider eight medicines, including Tamiflu.
swissinfo with agencies
Tamiflu was invented by Gilead and licensed to Roche in 1996.
The oral treatment has proven effective against influenza A and B and the avian H5N1 strain of influenza currently circulating in the Far East.
In preparation for a possible bird flu pandemic, governments are stockpiling the drug. So far Roche has received bulk orders from around 50 countries.