A study published in late July found poor adherence among children in London who took Tamiflu for prevention of pandemic H1N1 in the spring.

Less than half the grade-school-age children and 76 percent of the 13- and 14-year-old students completed a full course of medication.

More than half the children reported side effects, such as nausea, stomach cramps and trouble sleeping. Almost 1 in 5 reported a neuropsychiatric side effect, such as poor concentration, confusion or bad dreams, even though the U.S. Food and Drug Administration says neuropsychiatric side effects are rare.

Moreover, a new study found that Tamiflu and Relenza are unlikely to prevent complications such as asthma flare-ups or ear infections in children who have seasonal influenza. But they do increase the risk of vomiting.

The authors of the study, published in the British Medical Journal, said they didn't know if their findings could be generalized to the pandemic flu strain.

Antiviral drugs can be underused as well as overused, Schechter said. Some Californians who died from novel H1N1 influenza did not receive antivirals.

``I'm afraid the medications are not being used in some instances where they should,'' he said. ``But there are also international reports of resistance developing. Both of those extremes are concerning.''

A handful of resistant H1N1 cases have been reported among people who took Tamiflu preventively: three in Japan and one each in Canada, Hong Kong and Denmark.

Those cases are not surprising nor of great concern to health authorities, said Dr. Tim Uyeki, a medical epidemiologist with the CDC. They are cropping up sporadically and don't seem to be spreading from person to person.

``The most important question for public health is not whether sporadic cases occur but whether there is ongoing transmission of oseltamivir-resistant strains,'' Uyeki said.

A study published in March about the spread of the H5N1 avian flu, which has been circulating in bird flocks in recent years and has killed 262 people, showed the virus rapidly developed resistance to a different class of antiviral drugs, adamantanes.

``With bird flu, we found some resistance started in China and spread throughout the world in a few years,'' said study author Daniel Janies, an evolutionary biologist at Ohio State University. ``Overuse contributes to resistance. Basic natural selection predicts it. We can demonstrate why you should not use these drugs unless you have to.''

Other antiviral drugs exist, but the pandemic H1N1 virus is resistant to the adamantane class. If it develops resistance to Tamiflu also, only Relenza would be left to treat the illness, barring the development of new antiviral medications.

Relenza, Schechter said, is indicated only for people age 5 or older. Used less commonly than Tamiflu, it is inhaled as a powder, and people who are seriously ill or have difficulties with breathing cannot take it.

``The more choices you have, the better for treatment,'' Schechter said. ``To lose any one of those options would pose great challenges for treatment of those who are most vulnerable or likely to die.''

Associated Press staff writer Grahame L. Jones contributed to this report.