Biogen Idec (NASDAQ:BIIB) saw substantial growth in the third quarter, but apparently it’s coming from all the wrong places. Getting a taste of Intuitive Surgical's (NASDAQ:ISRG) medicine, investors sent the Stock Advisor pick down more than 7% even though it beat analyst revenue and earnings projections.

Revenue was up 38% year over year. Sales of Biogen’s biggest seller, multiple sclerosis drug Avonex, were up 26%, and its share of revenue from Rituxun, which it sells with Genentech (NYSE:DNA), was up 27%.

So what was the problem? Well, revenue from multiple sclerosis drug Tysabri, which Biogen sells with Elan (NYSE:ELN), was up just 172% year over year. Actually, it was the growth in the number of patients on Tysabri that disappointed.

As fellow Fool Brian Lawler pointed out last quarter, the growth of patients coming onto Tysabri had already been slowing, but a report last summer of two new cases of a rare and often deadly brain disorder seems to have smacked down the growth of paying customers to just 11.5% quarter over quarter. Granted, that's still tremendous annualized growth, but it's not going to get Elan and Biogen to their goal of 100,000 patients on Tysabri by the end of 2010.

There’s some real risk that Tysabri won't be able to compete with other multiple sclerosis drugs -- like Teva Pharmaceuticals' (NASDAQ:TEVA) Copaxone, or Rebif from EMD Serono and Pfizer (NYSE:PFE) -- on a first-line setting. Nonetheless, there's no doubt that Tysabri works. After failing other treatments, patients will likely determine that the risk/reward benefit for Tysabri is good enough to start using it, and sales will eventually pick up again.

With Biogen trading at less than 13 times its expected adjusted 2008 earnings, the company is a real steal if it can continue to produce adjusted bottom-line growth in excess of 25%. While there's some risk of a slowdown, just like the risk/reward ratio for Tysabri, there seems to be a lot more reward potential than risk built into Biogen's stock price.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.