NEW YORK — Barr Laboratories Inc. on Thursday began shipping its version of Prozac, the hugely successful drug that revolutionized the treatment of depression, in the biggest generic drug launch ever.

        Indianapolis-based Eli Lilly and Co.'s last valid patent for Prozac expired Thursday, paving the way for the Food and Drug Administration to grant final approval of Barr's product. The generic should be available for sale today.

        Barr, which is in the midst of buying Cincinnati's Duramed Pharmaceuticals, won't specify how much the generic Prozac will cost, but said the price will be between 25 and 40 percent lower than the daily cost of $2.63. That means the approximately 2 million people taking Prozac will save up to $383 each annually. The drug is available by prescription only.

        Prozac generated $2.6 billion in sales for Lilly in 2000 and is the biggest selling drug to ever go off patent. It became a blockbuster because it lacks the harsh side effects of earlier depression medicines, an advancement that spurred more doctors to prescribe the medicine and consequently ended some of the taboo surrounding mental illness.

        A six-year battle over rights to Prozac ended last week when a U.S. district judge in Indianapolis issued an order invalidating a patent that would have protected Lilly's monopoly until December 2003.

        The judge's action followed the refusal of the U.S. Court of Appeals in Washington, D.C., to rehear Lilly's appeal. Lilly still plans to appeal to the U.S. Supreme Court but conceded that it is highly unlikely to succeed.

        “I feel like an expectant father. We've done all the work and now all we do is wait,” said Bruce Downey, Barr's chairman and chief executive said in an interview earlier this week.

        Barr's plant in Forest, Va., has been working 10 hours a day, six days a week, since February to manufacture the 150 million capsules for the launch of fluoxetine, the generic name for Prozac.

        It ordered about $4 million of new equipment for the facility and has a logistics strategy set up to fill orders within three days.

        Typically, it takes Barr three weeks to fully launch a drug.

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