This week, the FDA continued with its own version of March Madness, scheduling advisory panel hearings on numerous drugs and medical devices. GlaxoSmithKline's (NYSE:GSK) controversial flu treatment, Relenza, was the subject of one such hearing yesterday.

Relenza is primarily used to reduce flu symptoms. It is best known for being one of the two drugs (the other was Gilead Sciences' (NASDAQ:GILD) Tamiflu) that governments began stockpiling in case of a bird flu pandemic.

In a vote yesterday, an FDA advisory panel suggested that the Relenza label should be strengthened to include the possibility of abnormal behavior and hallucinations in those who take it.

Glaxo has had an interesting experience with Relenza at advisory panel hearings since its approval in 1999. Glaxo first filed a marketing application with the FDA for the drug after completing three late-stage clinical trials. Prior to its approval, an advisory committee meeting was held to discuss the drug's safety and efficacy. After an FDA reviewer recommended against the drug's approval, 13 out of 17 panel members voted that the drug failed to show efficacy and shouldn't be approved.

Advisory panel recommendations, however, are not binding on the FDA's decisions to approve or deny a drug. In a case opposite to what happened with Dendreon (NASDAQ:DNDN) earlier in the year, the FDA ignored the negative advisory panel vote about Relenza, eventually approving the drug anyway.

Sales of Relenza were $168 million last year and $372 million in the first nine months of 2007, thanks to growth from governments stockpiling the drug. With overall pharmaceutical sales of $37 billion last year, the added safety concerns to the Relenza label will have a modest impact (if any) on Glaxo's top line. Still, Relenza's history shows the unpredictability of the FDA, and what can happen when investors put too much stock in the outcome of an FDA advisory panel hearing.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. GlaxoSmithKline is an active Income Investor pick. The Fool has an A+ disclosure policy.