Biogen Idec (NASDAQ:BIIB) held its research and development day yesterday to update investors on its pipeline, but it's the research on countering a side effect of one of its already-marketed drugs that might have the potential to increase sales the most.

Growth in sales of Tysabri, Biogen's and Elan's (NYSE:ELN) multiple sclerosis drug, has slowed considerably since the drugmakers reported that patients taking the drug had come down with cases of a rare, potentially deadly brain infection called progressive multifocal leukoencephalopathy (PML). Tysabri was pulled off the market in 2005 because of reports of PML, but the FDA allowed it back on in 2006 because the risk-reward ratio is still skewed in the favor of helping patients.

And yet many patients aren't convinced. There are other treatments for multiple sclerosis available, like Biogen's Avonex, Teva Pharmaceutical's (NASDAQ:TEVA) Copaxone, or Rebif from EMD Serono and Pfizer (NYSE:PFE). These work well enough -- although probably not as well as Tysabri -- that many patients aren't willing to take the risk of PML.

Elan and Biogen can't make Tysabri any more effective, so the only way to get sales growing again at the previous feverish clip is to decrease the risk of PML. The companies are working on two fronts: trying to figure out how to detect PML so that it can be diagnosed early and how to treat patients once they get it.

Biogen hopes to have a test for the virus that causes PML available by the end of the year although, because the virus is present in many healthy individuals, it's not entirely clear if a test will be very useful. More importantly, the companies are working on a procedure for treating patients who have PML. The first step is a system for removing Tysabri from the blood stream -- Tysabri inhibits the mistaken attack of the body by the immune system that is MS. This inhibition is thought to allow the virus to induce PML. They're also testing a malaria drug sold by Roche to see if it's capable of treating the disease.

Ultimately it'll be how well patients survive PML, not the frequency of PML, that will help skew the risk-reward profile further in favor of Tysabri. So far five patients have gotten PML since Tysabri was brought back on the market, with one death.

With every drug, the side effect profile is going to be judged against its efficacy. In some cases, like Merck's (NYSE:MRK) Vioxx or Wyeth's (NYSE:WYE) Fen-phen, the side effects are high enough to get the drug kicked off the market. Fortunately for Biogen and Elan, the efficacy of Tysabri is good enough to justify keeping it on the market. Still anything that the companies can do to reduce the side effects is money well spent.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is an Inside Value recommendation. Elan is a Rule Breakers selection. The Fool's disclosure policy is a side effect of our integrity.