Looking for a stock with excellent near-term revenue growth potential and a 25.1% net margin over the trailing-12 months (per S&P Capital IQ)? Look no further than big biotech Biogen (BIIB 0.13%).

What makes Biogen even more tempting right now is that it seems a little ignored. Biogen sold off hard in March -- as did the rest of the sector -- but it hasn't participated equally in sector's comeback. And that's despite the FDA's recent nod to Biogen's new hemophilia drug Eloctate (peak sales estimate of $1.5 billion), and the strong launch of Biogen's blockbuster MS drug Tecfidera.

We all know that the time to buy a stock is when it's flying under the radar, and with Biogen's Q1 sales surging 51% year-over-year to $2.1 billion, is this a stock to buy?

Biotech investing is a chancy business, so it's worth digging deep to find this stock's biggest risk. Once you do, you'll find a fascinating story -- and one fraught with danger.

Could history repeat itself?
As individuals, we are all products of our past to a certain degree, and the same could be said of companies. That's why front-and-center in Biogen's outlining of 10-Q risks were the executives' major concerns about Tecfidera's launch.

The filings warn, "If we are unable to successfully execute on our commercialization plans for TECFIDERA, our future revenue growth ...may be adversely affected." Specifically, the filings state, "damage to our sales and reputation, and physician and patient confidence in Tecfidera relating to any adverse experiences or events that may occur with patients treated with Tecfidera."

It's a mistake to think those sentences are just standard securities filing legal jargon.

Biogen's execs are well aware of what happened the last time the company did a major launch of another highly promising and novel MS drug. Despite high expectations for that drug, the company's following year was bedeviled with nightmarish problems.

Tysabri's rough launch
Tysabri -- touted by some as a miracle drug -- won FDA approval in November, 2004. Less than a year later, the drug was forced off the market after several patients died from a rare brain infection known as PML. Tysabri wasn't relaunched until 2006, after Biogen developed a test used to predict if a prospective candidate might be vulnerable to PML.

An experience like this not only changes the commercial trajectory of a drug company short-term, it can have serious long-term effects on the company's reputation with doctors and patients. Biogen has been down a very difficult trail with new MS drugs; that's why they are sweating the launch of Tecfidera.

Biogen's battle with Novartis and Sanofi
Biogen has another problem with Tecfidera. The drug doesn't have the market to itself.

Besides Tecfidera, the most effective MS oral treatments are Novartis' (NVS 0.01%) Gilenya and Sanofi's (SNY -0.68%) Aubagio. Gilenya was tied to PML, when the FDA sent out a safety alert in August last year.

Sanofi's drug was pegged by AdverseEvents for probable safety-related action from the FDA in January. As reported by FiercePharma, the AdverseEvents "RxScore" -- a scale that indicates safety, with 100 representing the highest risk the drug scaled at 48. It should be noted side effects are sometimes not caused by the drugs being reported, as patients often take multiple drugs.

In addition, AdverseEvents reported a few days ago that two other MS drugs, Pfizer's MS drug Rebif and Novartis's Extavia, were both associated with depression and suicidal behavior.

Meanwhile, how are Tecfidera patients faring?
According to Jeffery Cohen, a neurologist at the Cleveland Clinic, the drug has some problems. "At least right after initiation there's more side effects," Dr. Cohen said. He reported that perhaps as many as 10% or 20% of patients discontinued the medication because of side effects.

AdverseEvents also reported a few days ago that while Tecfidera had a higher association with gastrointestinal upset than did its rivals, the drug had comparatively few reported adverse events.

You can almost hear the executives at Biogen sigh in relief.

Tecfidera cannibalizing Tysabri
Not so fast, though. It turns out that Biogen also faces the risk of Tecfidera cannibalizing sales of other drugs -- like Tysabri, for example, which brought in almost half a billion in first quarter 2014.

Multiple Sclerosis is a disease affecting nearly 400,000 people in the United States and 2.5 million globally. Right now, Biogen looks like it has brought an excellent new oral medication to market, one that could easily join the list of 2014 superstars.

But the ramp up of a blockbuster-to-be is always highly risky, and that's never more true than when the disease is MS. Biogen's fate will be the tale of two drugs in 2014--Tysabri and Tecfidera. Biogen bought out the full rights to Tysabri last year, despite continuing problems with Tysabri's safety profile, so investors need to keep a watchful eye on both of them.