Elan (NYSE:ELN) shareholders are a certifiably nervous lot these days. In a week, the Food and Drug Administration will hold hearings on whether -- and under what circumstances -- it will allow Tysabri, a treatment for multiple sclerosis that Elan developed and co-markets with Biogen IDEC (NASDAQ:BIIB), to return to the market. When the two companies voluntarily pulled the drug following reports linking it to two deaths from progressive multifocal leukoencephaly (mercifully abbreviated PML) last year, Elan's share price tanked by more than 70%. When another case came to light -- this time with a patient taking Tysabri as a monotherapy -- the losses from the peak increased to 90%.

Elan seemed cooked. Tysabri was its great hope. But there was no denying a few things: Tysabri was more effective in treating MS than anything else on the market, the two cases of PML presented themselves in patients who were using Tysabri in combination with other drugs, and MS sufferers and many neurologists alike were clamoring for its return to the market. In November, the FDA put Tysabri on priority review, and next week, they'll hold the meeting. To say that a lot rides on the outcome of this meeting for Elan shareholders is an understatement. The stock has risen 300% from its April lows, but recent weakness in the shares suggests that some shareholders are hedging their bets or are selling entirely.

So when, on Feb. 28, a University of Texas report stated that Tysabri can deplete immune-system cells by the same level as the HIV virus, Elan shares went into a complete tailspin, dropping more than 14% before recovering to finish down only 7% at the close.

However, Bloomberg said on Wednesday that this report, which has yet to be accepted by a medical journal, was circulated by a spokesman for rival MS drugmaker Teva Pharmaceutical (NASDAQ:TEVA). If this is true, I believe it's a pretty dastardly deed -- not so much because a study showing that Tysabri lowers the immune system as much as HIV does is slanderous, but because doing so shows that Teva is willing to try to influence proceedings before the FDA meeting by introducing additional doubt for the safety of Tysabri. It also shows that the company considers the drug to be a legitimate threat to its own MS treatment, Copaxone.

Let's face it -- HIV is scary, a true hot button if there ever was one. But the report doesn't say that Tysabri is as dangerous as HIV. It says that it lowers immune systems to similar levels. Well, hello! Tysabri is an immunosuppressant. This is how the drug combats MS. Essentially, Tysabri sequesters the immune system to keep it from destroying the central nervous system. The challenge is to keep the immune system from dropping too low. This, incidentally, is why most PML cases exist in people who have full-blown AIDS. The virus that causes PML exists in an estimated 70% of the general population, but remains latent.

Now, imagine that -- an immunosuppressant that suppresses the immune system! None of this implies that suppressing the immune system isn't an extremely dangerous thing to do. But it also happens to be the most effective way to treat a debilitating and deadly disease.

This is not to cast aspersions on the original Texas research, which may turn out to be bulletproof. But Tysabri is a drug administered once a month by doctors, who, one would assume, observe and adjust dosing as needed. But while the study may have its clinical use in helping make Tysabri (and future MS treatments) safer, the link to HIV in the public domain is spurious. You might as well say that helium is as dangerous as hydrogen because it also makes balloons float.

Makes you wonder why Teva's representative went to such an effort to make the study known, doesn't it?

Biogen is a Motley Fool Stock Advisor recommendation.

Bill Mann doesn't even play a doctor on TV. He does hold shares of Elan Pharmaceuticals. Bill is the co-advisor of the Motley Fool Hidden Gems newsletter, in which he and Tom Gardner monthly recommend stocks that the market has ignored, mistreated, and misunderstood. A 30-day guest pass is a click away!