(Adds detail, context)

By Samantha Kareen Nair

Sept 2 (Reuters) - The U.S. Food and Drug Administration approved Tesaro Inc's treatment for chemotherapy-induced nausea and vomiting in adults, the company said on Wednesday.

The drug, known chemically as rolapitant, was approved in combination with other agents to prevent delayed nausea and vomiting associated with some forms of chemotherapy, Tesaro said.

Chemotherapy-induced nausea and vomiting, considered one of the most acute side effects of cancer therapy, occurs in up to 80 percent of patients, according to the National Institute of Health.

Rolapitant's approval comes at a time when rival therapies, including Merck & Co Inc's oral drug, Emend, are losing patent protection.

As one of Merck's key oncology products, Emend brought in sales of about $553 million last year. While the U.S. patent for the oral formulation expired in April, the injectable remains protected until 2019.

Rolapitant, to be sold as Varubi, is Tesaro's first product to win FDA approval. It is expected to go on sale in the fourth quarter and will address a $1 billion market in the United States, the company said.

Laidlaw & Co analyst Yale Jen said he expects the drug to rake in peak annual sales of about $400 million.

Rolapitant works by blocking the activation of neurokinin (NK)-1 receptors in the nervous system, which play a role in nausea and vomiting.

Tesaro, which is also testing an intravenous (IV) formulation of rolapitant, licensed the drug from Opko Health Inc in December 2010, giving it worldwide rights to develop and market the therapy.

Tesaro's stock was up about 5.5 percent before the bell, while Opko's shares rose about 4.6 percent.

(Reporting by Samantha Kareen Nair in Bengaluru; Editing by Savio D'Souza and Anil D'Silva)