Generics drug manufacturers have lost a slice of their market with the reinstatement of a European patent covering Merck's multi-billion-dollar osteoporosis drug, Fosamax. The patent was originally revoked by the European Patent Office in 2004, but careful rewording saw the decision reversed on 28 March.

The patent covering the once-weekly dose of the $3 billion (£1.5 billion)-a-year drug now reads that alendronate (Fosamax) is used to treat osteoporosis. The invalidated patent had said that the drug treated the condition by preventing bone reabsorption.

A leading patent attorney told the Financial Times that he had never heard of a patent being reinstated when a product was already on the market.

The surprising European decision will only go some way to alleviate Merck's woes; the alendronate patent was invalidated in the US in 2005. The US appeals court turned it down in favour of Israeli generics drug maker Teva Pharmaceuticals. The US patent will disappear next year, wiping out about 20 per cent of the drugs estimated earnings for 2009.

Merck's new European patent applies in 23 European countries, but not in the UK, where Fosamax lost its basic patent protection in 2003.