The U.S. Food and Drug Administration has expanded the approved use of Tamiflu to treat children as young as 2 weeks old who have shown symptoms of flu for no longer than two days.

The drug is not approved to prevent flu infection in this population. In addition, the safety and efficacy of Tamiflu to treat flu infection has not been established in children younger than 2 weeks old, according to a press release.

Tamiflu was approved in 1999 to treat adults infected with flu who have shown symptoms for no longer than two days. It has since been approved to treat flu in children ages 1 year and older who have shown symptoms of flu for no longer than two days, as well as for adults and older children.

FDA officials have advised that patients who are younger than 1 year old should only be prescribed the medicine in smaller doses based on weight. These smaller doses will require a different dispenser than what is currently co-packaged with Tamiflu.

“Pharmacists must provide the proper dispenser when filling a prescription so parents can measure and administer the correct dose to their children,” said Dr. Edward Cox, director of the Office of Antimicrobial Products for the Center for Drug Evaluation and Research. “Parents and pediatricians must make sure children receive only the amount of Tamiflu appropriate for their weight.”

Tamiflu is the only product approved to treat flu infection for children younger than 12 months old.

According to the Centers for Disease Control and Prevention, children younger than 2 years old are at higher risk for developing complications from the flu, with the highest rates of hospitalization in those less than 6 months of age.

The FDA expanded the approved use of Tamiflu in children based on extrapolation of data from previous study results in adults and older children, as well as additional supporting safety and pharmacokinetic studies sponsored by both the National Institutes of Health and the Roche Group, Tamiflu’s manufacturer.

Data on how the drug is metabolized in the body indicated a dose of 3 milligrams taken twice daily provided concentrations of Tamiflu similar to those observed in older children and adults, and it is expected to provide similar efficacy in this very young age group, Cox said.

Almost all of the 135 pediatric patients enrolled in the two safety studies had confirmed flu. Results from these studies showed the safety profile in children younger than 1 year old was consistent with the established safety profile older patients.

The most common side effects reported with Tamiflu use in this age group include vomiting and diarrhea. Although not seen in the new studies, officials said, rare cases of severe rash, skin reactions, hallucinations, delirium, and abnormal behavior have been reported.

Even though the medicine has been approved for younger patients, officials said Tamiflu should be considered a substitute for early, annual flu vaccination, as recommended by the Advisory Committee on Immunization Practices.

Health officials recommend patients aged 6 months and older receive an annual flu vaccine.