NEW YORK, Jan 13 (Reuters Health) -- The Food and Drug Administration (FDA) has issued an advisory to doctors about the safe use of influenza drugs.

The federal agency also recommends that patients talk to their doctors about these drugs, and that patients read the prescribing information supplied with them. This information includes side effects noted during studies of the drugs before approval and, for two of the drugs, during the period after they went on the market.

The advisory reminds practitioners that to be effective, influenza drugs must be started during the first 2 days of symptoms, "and the demonstrated treatment benefit has been limited to a modest increase in the rate of symptom improvement."

In addition, FDA officials note that "antiviral drugs have not been proven to prevent or effectively treat viral complications of influenza, such as viral pneumonia."

The agency also notes that patients with compromised health may develop bacterial infections that mimic or that may accompany influenza that will not respond to antiviral drugs, but require treatment with antibiotics. "FDA has received reports of patients with serious bacterial infections who initially had influenza-like symptoms and who had progression of bacterial infection during treatment with antiviral drugs alone," according to a statement issued by FDA officials.

Four drugs have been approved for the treatment of uncomplicated influenza. Two -- amantadine (Symmetrel, among others) and rimantadine (Flumadine) are used against influenza A viruses, while two more recently approved drugs, zanamivir (Relenza) and oseltamivir (Tamiflu) are active against both influenza A and B strains.

The FDA also cautions healthcare professionals on the use of the antiviral drug zanamivir (Relenza) in patients with asthma or chronic obstructive pulmonary disease because of the risk of bronchospasm with the drug. "If a decision is made to use Relenza in patients with airway disease, this should be done only under careful supervision and with adjunctive care," such as making sure that bronchodilators, drugs that help to relieve tightened airways, are available.

Agency officials stress that "vaccination remains the primary method of preventing and controlling influenza."