US regulators have approved the first generic version of Pfizer’s blockbuster anti-depression drug Zoloft (sertraline), giving Teva Pharmaceutical Industries the green light to market its version exclusively in the USA for six months.

The Isreali drugmaker is planning to sell sertaline in 25mg, 50mg and 100mg tablets, with a market introduction scheduled for the end of this month, marking a move that could prove fairly lucrative to the firm, given that Zoloft pulled in sales of nearly $2.6 billion last year.

But on Friday Pfizer said it is planning to launch its own generic version Zoloft in the USA to help weather the competition from Teva’s copycat. This 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.

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.