Pfizer as said it plans to launch a generic version of its own antidepressant drug Zoloft in the USA in competition to Teva Pharmaceutical Industries.

Teva has 180 days’ marketing exclusivity for generic Zoloft (sertraline) under US regulations that reward the first company to file for approval of a copycat version of a branded product. The patent on Zoloft is due to expire today.

But Pfizer’s move reflects an increasing tendency by original drug developers to defend their franchises after they lose patent protection though the use of so-called ‘authorised generics’ which compete with the new entrant during the exclusivity period. Sometimes they are sold by the brandname company itself, though more commonly a third-part company is used – often to sidestep patent litigation cases.

Earlier this year, the US federal Trade Commission launched an investigation into authorised generics agreements, amid allegations that the practice is stifling competition by acting as a disincentive for generic companies to develop products. The FTC is also looking into the practice of pharmaceutical companies paying rivals to keep generics off the market.

Sertraline, which generated US sales of almost $2.6 billion last year, promises to be a lucrative product for Teva, although the availability of another generic version will almost certainly reduce the rewards of an early launch for the Israeli firm.