In a bit of good news for big pharma, Wyeth (NYSE: WYE) received marketing approval for its second-generation version of its blockbuster antidepressant Effexor on Friday.

The new version of Effexor, named Pristiq, is Wyeth's attempt at extending the sales lifecycle of its Effexor franchise, which begins to face patent expirations this year. Sales of its Effexor franchise were $3.8 billion last year, and it competes against drugs like Forest Labs' (NYSE: FRX) Lexapro and Pfizer's (NYSE: PFE) Zoloft, which is available as a generic. Wyeth had been trying to get Pristiq approved since first filing for FDA approval in 2006 and receiving an approvable letter for the drug in early 2007.

Another big FDA win for Wyeth occurred 11 days ago when it received approval for another second-generation product in its hemophilia A treatment Xyntha. Wyeth's already approved hemophilia drug Refacto had sales of $335 million last year and is facing patent expiration in 2010.

Getting second-generation drugs approved to replace their hit predecessor drugs is a mainstay of extending the sales life of many big pharma drugs. This can be an effective strategy when it works, but often fails if private insurers and other payers are unwilling to pay more for the "improved" second-generation product vs. cheaper generic competitors of the first-generation compound.

Only time will tell which category these two Wyeth drugs fit into.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. Pfizer is a pick of both the Inside Value and Income Investor newsletters. The Fool has an A+ disclosure policy.