Prozac has a new lease on life.

The Food and Drug Administration recently approved the well-known antidepressant in combination with Zyprexa for the treatment of bipolar depression. Eli Lilly (NYSE:LLY), which markets both drugs, calls the combination the first to treat only the depressive phase of the disorder and expects the drug on pharmacy shelves in mid-January.

About 2.5 million people in the U.S. have been diagnosed with bipolar disorder. Lilly claims that the true number of sufferers could be closer to 10 million.

Also according to Lilly, the use of antidepressants alone in individuals with bipolar disorder can spur manic phases that can result in suicide -- underlining the importance of this medication to treat what the company calls an "unmet" medical need.

Currently, Prozac is used for depression, while Zyprexa is used for schizophrenia as well as the short-term treatment of manic episodes. (Incidentally, Zyprexa is also under consideration by the FDA for long-term maintenance of bipolar disorder.)

But Prozac, itself so huge a blockbuster it's become part of the vernacular, lost patent protection several years ago and fell prey to copycats. Competition from other antidepressants such as Pfizer's (NYSE:PFE) Zoloft and GlaxoSmithKline's (NYSE:GSK) Paxil didn't help, either.

Zyprexa, meanwhile, Lilly's new star, has been bringing in more than $4 billion in revenues, but sales are slipping. Indeed, a study released in November showed a far cheaper drug to be as effective in schizophrenia.

Good as the prognosis sounds for Symbyax, some health professionals fear side effects, which might translate into barriers to acceptance. And while the company cites an unmet medical need, doctors are already prescribing combinations of other drugs to stabilize the moods of bipolar patients.

Still, even if Symbyax isn't the next Prozac, it can certainly help boost Zyprexa sales. And it might just give Prozac a whole new reason to live.

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