One quarter doesn't make a trend, but considering how much Tysabri means to Biogen Idec (Nasdaq: BIIB) and Elan (NYSE: ELN), the number of patients added in the most recent quarter should worry investors at least a little.

 

Q4 2009

Q1 2010

Q2 2010

Q3 2010

Q4 2010

Patients on Tysabri as reported at end of the quarter48,80050,30052,70055,10056,600
Patients added during quarter2,6001,5002,4002,4001,500
Quarter-over-quarter growth5.6%3.1%4.8%4.6%2.7%

Source: Company releases. Patient additions and growth based on initial reports of patient numbers.

Patients coming onto Tysabri bounced back after a relatively poor first quarter of 2010, so the most recent quarter could be a similar fluke. Whether Biogen and Elan will be able to reaccelerate growth basically comes down to whether the decrease was because of competition from Novartis' (NYSE: NVS) new oral multiple sclerosis drug Gilenya or whether patients are still freaked out about the chance of getting progressive multifocal leukoencephalopathy (PML) -- a potentially deadly brain infection.

We should get some sense of whether it's the former when Teva Pharmaceuticals (Nasdaq: TEVA), Biogen, Merck KGaA, and Bayer release fourth-quarter earnings in the coming weeks. If sales of their multiple sclerosis drugs are also slipping, we'll know that Gilenya is stealing patients away from the drugs that have to be taken via injection.

Biogen and Elan should be able to handle the PML issue much easier. The drugmakers have developed a test for the virus that causes PML after Tysabri suppresses the immune system, which should be available later this year. Patients who don't have the dormant virus in their system aren't likely to get PML, which should give them increased confidence in taking the drug. Biogen's management even commented that the test might end up being a slight loss leader for the company.

It'll certainly be able to make up the loss many times over if it can get the growth rate of Tysabri back up.

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Elan is a Motley Fool Rule Breakers choice. Novartis is a Motley Fool Global Gains selection. The Fool owns shares of Teva Pharmaceutical Industries. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.