This past Monday I knew I would be writing this article. I knew this was coming, I just didn't know precisely when. On that day, Biogen IDEC (NASDAQ:BIIB) and Elan (NYSE:ELN) pulled the multiple sclerosis drug Tysabri off the market because two patients using Tysabri in combination with Avonex had a rare viral infection called PML, and one of these patients has died.

Since Tysabri was widely believed to be a major medical breakthrough for MS patients, it was expected to become a huge blockbuster drug for the two companies. So its removal from the market led to very severe stock price declines for both Biogen IDEC and Elan.

On Monday while I was listening to the conference calls and trying to absorb all the information, I was wondering just how long it would take before these companies were sued. I was expecting it could take a few days, but it wouldn't be long. Sure enough, it took a whole two days before the first class action suit was filed against Biogen IDEC. And just like cockroaches, when you find one, you find dozens. So, true to form, this opening salvo was quickly followed by additional suits.

And so it goes. So how did I know this was going to happen? It's because these class action suits are an epidemic in the drug industry, with nearly one-third of biotech companies currently named as defendants. If a company has a setback with one of its drugs and a significant stock decline results, you can expect that within days the class action suits will start flying. It's as reliable as the eruptions of Old Faithful.

As with most of the complaints I've read, the allegations here are way off the mark. Consider that the FDA only gives a Priority Review (faster approval than normal) after the company has submitted the clinical data and the FDA has had a chance to look it over. The FDA approved Tysabri under the Priority Review designation. This is a pretty strong endorsement that a drug is important in the disease it is treating. To say that the stock prices of these companies were inflated due to intentionally misleading statements from management is bunk. The stock prices were where they were because the FDA felt this was an important drug and investors picked up on that.

All drugs have side effects. That is the tradeoff for getting the therapeutic benefit. Sometimes these side effects are benign and sometimes they are severe. Sometimes they are common and sometimes they are rare. No one knows what is going to happen when a drug, or a combination of drugs, is used. So when something bad happens, it is an unfortunate event, but it doesn't automatically mean that management was recklessly misleading investors.

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Fool contributor Charly Travers is the biotech analyst for Motley Fool Rule Breakers . He does not own shares of any company mentioned in this article. The Motley Fool has a disclosure policy .