India's top drugs firm, Ranbaxy Laboratories, has reported a 79% jump in profits, thanks to surging exports of a generic antibiotic to the US.

Ranbaxy beat expectations with a net profit of 1.6bn rupees (£21m; $33m) in the three months to end-September, up from 891m rupees a year earlier.

Ranbaxy's shares fell slightly on the announcement, largely owing to concerns that some of its subsidiaries may be underperforming, but they have nonetheless risen by one-third since the beginning of the year.

The said exports for the quarter nearly doubled to 5.2bn rupees, with sales of a cheap generic copy of GlaxoSmithKline's Ceftin antibiotic earning $75m since its US launch in March.

Indian pharmaceutical firms, lacking significant global brands of their own, have prospered in recent years by copying well-known drugs whose patents have lapsed.

US success

Ranbaxy now claims a market share of 87% of new prescriptions for Ceftin.

Campaigners in the US are arguing for greater access to generics, something often blocked by the courts.

Earlier this week, rival Indian producer Dr Reddy was prevented from selling a version of AstraZeneca's lucrative Prilosec heartburn remedy.

So far, however, Ranbaxy has had few such problems.

Managing director DS Brar said that the firm was reaching "the desired critical mass" in the US.