Israeli drugmaker Teva Pharmaceutical Industries and India’s Ranbaxy have obtained the go ahead from US regulators to put their generic versions of Merck and Co’s billion-dollar cholesterol buster Zocor on the market.

The US Food and Drug Administration cleared Teva and Ranbaxy’s versions of the drug on Friday, effectively allowing the companies joint exclusivity for six months. Although the agency initially turned down the claim for joint exclusivity, the decision was overturned by a judge and so stays valid while the FDA considers an appeal.

The approval also follows a failed legal bid for a temporary restraining order by Sandoz - the generic arm of Swiss drug giant Novartis – which is preparing its own copycat version of Teva for the US market and had hoped to delay the market introduction of rival products.

Zocor (simvastatin) lost its patent protection on June 23, opening up a $3.14 billion US market to generic rivals. The drug, a star performer in the Merck’s portfolio and the second biggest-selling cholesterol drug in the world, raked in sales of $4.4 billion last year, and the group has previously forecast a decline to $2.3-$2.6 billion for 2006 because of generic erosion.

But in a surprise move, reports emerged last week of a deal between Merck and US insurance giant United Health that would see patients paying less for Zocor than traditionally cheaper copycat versions, sparking a new angst in the generic sector that companies are shifting up a gear the battle to protect their brands.

Another tactic employed by Merck was to sign up Indian drugmaker Dr Reddy’s for an authorised copycat version of Zocor and its prostate cancer drug Proscar (finasteride), which was approved by the FDA last week, in a move that allows the branded major at least some measure of control over the generic erosion of its top products as well as the chance to sidestep expensive litigation cases.