Thanks to what a Pfizer Inc spokesperson calls a clerical error, the giant drug company might lose its monopoly on Norvasc, a blockbuster drug used to treat hypertension. In 2002, Pfizer received a letter from Mylan Laboratories Inc. The drugmaker let Pfizer know that it had just asked the Food and Drug Administration for approval to sell generic Norvasc. Under the law, Pfizer had 45 days to sue Mylan for patent infringement and receive a 30-month stay, preventing the FDA from approving Mylan's application. Those 45 days came and went. Finally on September 20, 2002--day 59--Pfizer's lawyers at Kaye Scholer and Thorp Reed & Armstrong filed suit against Mylan in federal district court in Pittsburgh.

Lawyers for Pfizer would not comment on the mistake, and Pfizer spokesperson Bryant Haskins wouldn't elaborate. But by October 2002, Pfizer was scrambling to keep the FDA from approving Mylan's generic: In a press release, the company told investors that it was looking for a way to stop the FDA. Apparently there wasn't. Mylan got the okay in October 2005--nearly two years before Norvasc's patent is set to expire.

Still Mylan isn't rushing generic Norvasc into drugstores. Pfizer's suit against the company is scheduled for an October 2006 trial. If Mylan markets generic Norvasc and loses the suit, it would be on the hook for astronomically high, treble damages. Not only would the company have to pay back any profits it made, but it would also have to pay Pfizer for lost profits, which would be much higher than any revenue Mylan could earn from sales of a low-priced generic. In 2005 Norvasc sales were almost twice Mylan's earnings for its 2006 fiscal year. A Mylan spokesperson says the company is aware of the risk and has not yet launched the drug. In counterclaims against Pfizer, Mylan says that two Norvasc patents are invalid. Mylan's lawyers at Rothwell, Figg, Ernst & Manbeck declined comment.

Pfizer's not the first drug company to miss the 45-day deadline. Patent litigators uninvolved with the litigation say that it's not unheard of for "Paragraph IV" certification letters--the alerts that generic manufacturers are required to send to brand-name drug companies--to get lost inside their labyrinthine destinations. "It's a lawyer's nightmare, quite frankly," says Finnegan, Henderson, Farabow, Garrett & Dunner partner Robert Bajefsky, who has represented GlaxoSmithKline plc and Wyeth Pharmaceuticals.

But sometimes Pfizer has deliberately not sued within the 45-day period. That appears to be what happened with Pfizer's billion-dollar antidepressant Zoloft. Teva Pharmaceuticals USA asked the FDA for approval to market generic Zoloft in July 2002. Pfizer did not file a lawsuit. Afraid of marketing generic Zoloft and getting slapped with damages, Teva went to federal district court in Massachusetts seeking a declaratory judgment that it did not infringe Zoloft's patent. The tactic failed. The district court ruled that because there was no threat of Pfizer suing Teva, the court had no jurisdiction in the case. The U.S. Court of Appeals for the Federal Circuit upheld the ruling, and generic Zoloft stayed off the market. In 2005 Zoloft's global sales totalled $3.1 billion. Pfizer's patent expired in June, and this summer Teva said it would launch its generic version of Zoloft sometime during the third quarter. Of the case, Pfizer's Haskins would only say, "We look at each case on its individual merits and do not take legal action in every situation." Teva's attorneys declined comment, and Teva did not return calls.

Weston, Ontario-based Apotex Inc. suffered the same fate in its own quest to market generic Zoloft. Apotex also went to court seeking a declaratory judgment, but was denied. The company has appealed to the U.S. Supreme Court.

Meanwhile, Pfizer and its lawyers at Kaye Scholer have been fending off more Norvasc challenges. At press time a decision was pending in a trial against Synthon Laboratories, Inc, which also applied to make generic Norvasc. Pfizer has also succeeded in blocking Dr. Reddy's Laboratories Limited from making the drug, getting the Federal Circuit to overturn a New Jersey federal district court's ruling that allowed Dr. Reddy's to manufacture a variation of Norvasc. And in January, Pfizer stopped a Norvasc challenge from Apotex when a federal district court judge in Illinois ruled that Norvasc's patent was valid. Now, if only Pfizer could sell a drug to help with litigation stress.