Merck & Co. on Thursday said the Food and Drug Administration had approved its Singulair medication to treat the symptoms of perennial allergic rhinitis, or indoor allergies.

Singulair was approved to treat symptoms of seasonal allergies in 2003.

The once-a-day tablet blocks leukotrienes, an underlying contributor to allergy symptoms, while most allergy medications on the market block histamine, Merck said. Allergic rhinitis is commonly triggered by allergens such as dust, pet dander or mold.

About 50 million people in the U.S. are affected by some form of the condition, Merck said.

The drug, which also is approved to treat asthma, is sold in a tablet form for adults, as a chewable tablet for older children and in oral granules for children from 6 months to 5 years old.

Merck shares fell 15 cents to close at $30.41 Thursday on the New York Stock Exchange.