Oftentimes, pharmaceutical companies develop highly efficacious drugs to help treat diseases like multiple sclerosis (MS), Alzheimer's, or cancer, but the drugs never make it to market. Sometimes the problem occurs when the drug works too broadly and not only fights the disease but also gives the patient unpleasant or dangerous side effects.

For many drugs, this has long been the case, and often patients choose no treatment and a shorter, better-quality life over the longer life and terrible side effects that a drug might provide. Unfortunately, with the results that Genzyme (NASDAQ:GENZ) published today for its drug Campath for MS, this appears to be the case.

MS is a very difficult-to-treat disease with an unknown cause. This makes treating it very challenging and is comparable to trying to treat diseases like Alzheimer's and Parkinson's. As an aside: Tysabri for MS was discovered by Elan (NASDAQ:ELN) scientists looking for possible treatments for Parkinson's disease.

In September of 2005, in a study comparing Campath with Serono's (NYSE:SRA) Rebif for MS, Genzyme halted the trial after three patients developed a treatable but sometimes fatal immune response as a result of taking Campath. Later it was found that a total of six patients developed this immune response, and one patient unfortunately died as a result of it.

The good news from the results released today is that Campath appears to be more efficacious than Rebif in treating MS. The bad news is that this immune response seen in these patients could very well keep the drug off the market for this indication, since MS is a chronic condition, and patients who take drugs to treat this disease will need to take them for years or decades.

If six out of the 220 or so patients who took Campath (assuming a 2:1 Campath to Rebif ratio in the trial) came down with a potentially life-threatening disorder because of the drug, then imagine how many patients might become affected with this potentially deadly immune response over several years' worth of Campath dosing. This sort of adverse event profile is simply not acceptable for a drug that treats a chronic condition. Because of this, Campath will become at most a third-line or last-resort treatment for multiple sclerosis if it gets approved at all for this disease, which I highly doubt.

Campath is already approved in the U.S. to treat leukemia and is marketed by Schering AG. The results that Genzyme posted today will probably drive some off-label use of the drug in MS patients who are suffering greatly from the disease and for whom other therapies are not effective.

Even if Campath can eventually gain regulatory approval as a treatment for MS, it will take at minimum another four years for approval if you count the two phase 3 trials and the time it will take to file an application for approval with the FDA. As much as I like Genzyme, both the company and the stock, I won't hold my breath for this to occur.

Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy .