April 1 (Reuters) - France's Stallergenes said on Tuesday it received the U.S. Food and Drug Administration's green light to sell its immunotherapy pill Oralair for treating grass allergies.

The drug is a fast-dissolving tablet to be placed under the tongue that contains extracts from five grass pollens: sweet vernal, orchard, perennial rye, timothy and Kentucky bluegrass. It harnesses the immune system to alleviate allergies and is an alternative to current injectable treatments.

Oralair is the first oral immunotherapy drug for treating grass allergies to be approved in the United States. Merck and Danish partner ALK Abello are expected to launch their rival therapy Grastek this year too. In December, both drugs received strong recommendations from outside advisers to the FDA.

Oralair was approved in the European Union in 2008 and is also sold in Canada, Australia and Russia for the treatment of grass pollen allergy. It generated revenue of 22.2 million euros ($30.5 million) in 2013, up 37 percent from 2012.

Greer Laboratories, Stallergenes' U.S. partner, aims to begin selling Oralair within weeks of the FDA's go-ahead. But the peak season for Oralair prescriptions is between December and June, so U.S. revenue from the drug this year should be limited, Stallergenes' former CEO said last month. ($1 = 0.7271 Euros) (Reporting by Susan Kelly in Chicago; Editing by Eric Walsh)