With 44% of Amgen's (NASDAQ:AMGN) second-quarter revenue coming from its anemia drugs Epogen and Aranesp, even modest changes to the drugs' product labels can have big effects on Amgen's income statement and ultimately its share price.

So investors will want to pay attention to an FDA advisory panel meeting scheduled for tomorrow to discuss new labeling recommendations for the drugs known in medical circles as erythropoiesis-stimulating agents (ESAs). In particular, the panel will be discussing how to clarify the treatment guidelines when using the drugs to help patients with chronic kidney failure. It looks to be a murky topic.  

Why care about the findings of a panel that lacks the authority to set rules? Well, because the FDA will take the recommendations from the panel into consideration when deciding how harshly it will revise the marketing labels for Epogen and Aranesp.

After reading the FDA and Amgen briefing documents, one outcome stands out as a sure thing: The agency will cut out a substantial portion of the quality-of-life claims on the Epogen label. Both Amgen and the FDA state that this is going to happen because of insufficient proof of most quality-of-life benefits.

The bigger but still unresolved issue is how the FDA will deal with treatment guidelines for the ESAs in patients with chronic kidney failure.

The data the FDA presented in its briefing documents show contradictory statistical information for Epogen and Aranesp, which will make it hard for the advisory panel to recommend clear treatment guidelines. In other words, without solid data on how much of the drugs to use in treating patients with kidney failure, doctors are likely to cut back on their use of the compounds in this indication in the future.

U.S. sales of Aranesp already fell 19% year over year in the second quarter, thanks to stricter treatment guidelines in its use for cancer patients, but Epogen sales were still up for the quarter. With the label changes and likely new treatment guidelines, sales of Epogen (which Amgen can only market on-label for kidney failure patients) will start to feel the pain after the advisory committee meeting.   

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. An advisory panel has recommended the Fool's disclosure policy.