WASHINGTON (Reuters) - The U.S. Food and Drug Administration on Friday said it granted 10 drugmakers marketing approvals for the first generic forms of Singulair, the $5 billion-a-year asthma and allergy drug that is Merck & Co Inc's best-selling product.

The drugmakers, whose cheaper versions of Singulair are expected to quickly steal two thirds of the branded product's sales, include Mylan Inc, Teva Pharmaceutical Industries Ltd and the Sandoz division of Novartis AG.

Singulair, unlike most allergy drugs which block the protein histamine, instead blocks proteins called leukotrienes that cause inflammation.

Merck is counting on its respected pipeline of experimental drugs to come through and help offset the vanishing Singulair sales. Over the next 18 months, Merck aims to seek six drug approvals, including marketing applications for new types of therapies for insomnia and osteoporosis.

Shares of Merck were up 0.5 percent in late-afternoon trading, trailing a 1.3 percent advance for the Arca Pharmaceutical Index of large U.S. and European drugmakers.

(Reporting by David Morgan; Editing by Carol Bishopric and Leslie Adler)