FRANKLIN LAKES, N.J. (AP) - A generic version of the anti-depressant Prozac is catching on quickly, according to pharmacy benefits manager Merck-Medco.

Barr Laboratories offered the first generic version of the drug when Eli Lilly lost its patent protection Aug. 2. In the first week the generic was available, it was substituted for Prozac in 80 percent of the prescriptions sold through Merck-Medco's mail-service pharmacy.

Data for retail pharmacies in its network won't be available for a few weeks, Merck-Medco spokesman Jeffrey Simek said Monday. He said people using mail prescriptions usually switch to generic drugs faster than others.

Simek said the generic version of Prozac costs 20 to 40 percent less than the original, which is among the nation's top-selling drugs.

Patients with prescription plans often have an incentive to use generic drugs because their copayments are lower. Some states also require that generic drugs are substituted for the name-brand versions when they are available, unless doctors prescribe name brands.

Merck-Medco, which manages prescription programs for more than 65 million patients in the U.S., made a big push to get doctors and patients to shift to the generic drug. The company says it contacted 25,000 physicians who most often prescribe Prozac to tell them about the new version.

In trading on the New York Stock Exchange, shares of Pomona, N.Y.-based Barr Laboratories were up 65 cents to $85.80, while shares of Lilly were up $1.32 to $81.12.

Shares of Whitehouse Station, N.J.-based Merck & Co., which owns Merck-Medco, were up $1.40 to $70.55.

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