Washington ? Antibiotics soon will bear a big new warning — that overusing them makes them less effective.

Doctors must be sure a patient is suffering a bacterial infection, not a virus with similar symptoms, before prescribing antibiotics, say the warnings mandated Wednesday by the Food and Drug Administration.

Too often, doctors prescribe antibiotics for children with earaches caused by viruses, or for adults with colds or viral coughs.

Antibiotics have no effect on viruses; they only fight bacteria. Yet the government estimates that half of the 100 million antibiotic prescriptions written in physician offices each year are unnecessary.

The FDA also will require antibiotics to bear warnings reminding doctors to teach patients how crucial it is to take the drugs exactly as directed. Patients should not share pills with anyone else or stop taking them too soon just because they feel better.

Germs are growing increasingly impervious to antibiotics. Many common infections no longer are treatable with old standbys like penicillin, and some have become untreatable by every antibiotic on the market. Anytime antibiotics are used, survivor germs can emerge stronger and spread.