Having a strong legal team is one of the most important parts of a good generic drugmaker's operations. Being the first to challenge a branded drug's patents can mean billions of dollars, with the six months of marketing exclusivity that a generic drugmaker gets if it is successful in the courts. Mylan Laboratories (NYSE:MYL) got a big win this week when another generic drugmaker, Apotex, successfully invalidated patents on Pfizer's (NYSE:PFE) second-most-important drug and cleared the way for Mylan to launch its generic version.

Pfizer had been in court for separate cases with Mylan and Apotex over the validity of the patents for its hypertension drug, Norvasc. Losing the court case to Apotex and having its patents on Norvasc declared invalid means that Mylan is free to launch its generic version of the drug. Since Mylan was the first to challenge the Pfizer patents, it gets the six months of marketing exclusivity in the United States. This is a minor loss for Pfizer, since it just means generic entrants six months earlier than if the patents had been deemed valid and expired on their own in September of this year.

Sales of Norvasc were estimated at $2.7 billion in the U.S. last year. Pfizer's revenue from Norvasc won't drop to zero with the patent loss, but should decrease rapidly. For example, Merck's blockbuster $4 billion-a-year cholesterol medication, Zocor, experienced an 80% drop in sales after six months of facing generic competition last year. A similar trend should hold for Norvasc as well. While losing $1 billion or $2 billion in sales sooner than expected may sound like a big deal, for Pfizer, it is just another leak in the top line at the moment.

For Mylan, though, this is a big win. The company announced today that it had launched its generic version of Norvasc. The market exclusivity period could double the revenue Mylan brings in this year versus its 2006 fiscal-year revenue of $1.3 billion. Even better, because of the exclusivity period, Mylan's revenue will be relatively high-margin compared to its other generic products, which face much more competition and are less premium-priced.

Pfizer is an Inside Value recommendation. Are you looking for irresistible values? Check out Philip Durell's latest value picks by taking a 30-day free trial of the newsletter.

Fool contributor Brian Lawler is a generic kind of guy and does not own shares of any company mentioned in this article. Merck is a former Income Investor selection. The Fool has a disclosure policy.