The Food and Drug Administration said Wednesday that GlaxoSmithKline's ( GSK) antiviral drug Relenza could be used for influenza prevention among children 5 years and older and adults. The announcement, in the waning days of the current flu season, expands the use of Relenza, which is sold as a treatment for people who are infected by the flu. Relenza joins Tamiflu, from Roche and Gilead Sciences ( GILD), as antiviral drugs that can be used to both treat the flu and prevent it. Relenza is "not a substitute for the flu vaccine, which is the primary means for preventing influenza," the FDA said. "Consumers should continue receiving an annual flu vaccination according to guidelines on immunization practices." The inhalable Relenza is not recommended for people with asthma or chronic obstructive pulmonary disease, the FDA added. "Breathing problems, including deaths, were reported in some patients after the initial approval of Relenza," the agency said. "Most of these patients had asthma or chronic obstructive pulmonary disease." The announcement didn't do much for GlaxoSmithKline's stock, which was off 45 cents to $52.20 in late morning trading. GlaxoSmithKline last week asked the FDA to approve an experimental vaccine called FluLaval to treat adults 18 years and older for the 2006-2007 flu season. FluLaval comes from the company's acquisition last year of the Canadian vaccine maker ID Biomedical. During the 2005-2006 U.S. flu season GlaxoSmithKline sold about 7.6 million doses of another vaccine, Fluarix. If the FDA approves FluLaval, the company said it will makes as much as 30 million doses of both vaccines during the coming season.