A U.S. judge has upheld a patent for Merck & Co.'s best-selling product, the asthma and allergy medication Singulair, handing Merck a victory in its battle to ward off early generic competition for the drug.

Wednesday's ruling by Judge Garrett Brown in federal court in Trenton, N.J., means that Teva Pharmaceutical Industries Ltd., the Israeli generics manufacturer that challenged the Singulair patent, won't be able to start selling copycat versions in the U.S. until the expiration of patent protection in 2012. Teva could, however, appeal the decision in an effort to sell copies before then.

Singulair was the ninth best-selling prescription drug in the world last year, according to drug-data provider IMS Health. For the six months ended June 30, world-wide sales rose 6% to $2.3 billion. U.S. sales--those directly at stake in the Teva patent dispute--rose 6% to $1.5 billion in the first half of 2009.

Many analysts and legal observers thought Merck had the upper hand in the case because Singulair was an innovative approach to treating asthma when it hit the market in 1998.

But at a February trial in Trenton, Teva lawyers argued that the patent for Singulair was invalid because prior research would have taught anyone skilled in the art of drug development how to invent Singulair. Under patent law, so-called prior art can be grounds for invalidating a patent's claims. Teva also argued the patent was unenforceable because Merck misled the U.S. Patent and Trademark Office when it applied for the patent.