While combination drugs usually involve two drugs made by the same company, pairings of drugs from different makers, such as the venture between AstraZeneca and Abbott, are becoming increasingly common, says Holger Rovini, an analyst with Datamonitor in London.

Vytorin, introduced in the U.S. in 2004, generated sales of $2.43 billion worldwide last year, including $1.2 billion in the U.S. That's up more than 104 percent from 2004.

A combination drug does not extend the patent of the underlying component drugs. In Pfizer's case, Lipitor's patent will expire in 2010 and Norvasc's in 2007.

But the new combination receives its own patent, staving off generic competition. Because it may be more effective than the generic version of the component drug, the drug companies can usually charge a premium for it.

Combination drugs have proliferated in fields like the statin market, and in the treatment of diabetes and HIV -- all areas in which patients take medication on an ongoing basis and juggle taking multiple pills.

They can be a distinct advantage for patients, says David McCaffrey, director of the Center for Pharmaceutical Marketing and Management at the University of Mississippi. They require a single insurance co-payment instead of two and sometimes cost less than the two pills purchased separately, McCaffrey says.

"They offer patient convenience," says Dr. James Hopkins, a cardiologist at Christiana Care Health System, who has prescribed Vytorin to some patients.

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