Facing a pipeline that can’t compensate for the looming patent expirations of its biggest products, Wyeth hopes to generate revenue with a special introductory offer for it Pristiq antidepressant - when the pill launches next month, the price will be up to 20 percent less than the cost of Wyeth’s existing Effexor antidepressant. Pristiq is, essentially, a revamped version of Effexor XR, a $3.8 billion seller that loses patent protection in 2010, if not sooner.

And so Wyeth’s senior vp, Joe Mahady, told analysts that Pristiq will sell for a flat $3.41 per pill for both the mid-range and high-dose versions, according to Dow Jones. By comparison, the 150 mg version of Effexor XR costs $4.19 each, according to drugstore.com. That works out to a 22 percent difference. The site quotes the price for the 75 mg Effexor XR as $3.58 per pill, which is 5 percent more than the proposed price for Pristiq.

The gambit, of course, reflects the difficultes the myriad Wyeth faces - not only compensating for declining revenues and building market share for a new product, but trying to distinguish Pristiq from Effexor XR over the next two years. And then, of course, there are those other antidepressants that sell for still less than either Wyeth pill.

As Barbara Ryan, a Deutsche Bank analyst, writes in an investor note today: “We have long believed that the new drug pipeline at Wyeth is unattractive, and certainly insufficient to offset the loss of exclusivity through ‘10 for two of its largest drugs, Effexor XR and Protonix, which represent 35 percent to 40 percent of profits. The downside risks to Wyeth’s shares include a disappointing Pristiq launch this year and additional pipeline disappointments.”

Hat tip to the WSJ Health blog