The Food and Drug Administration on Thursday approved a new irritable bowel syndrome and constipation drug developed by Ironwood Pharmaceuticals Inc. and Forest Laboratories Inc.

The agency approved Linzess as a treatment for irritable bowel syndrome with constipation and for chronic constipation that is not helped by other treatments. Both conditions are believed to affect tens of millions of people in the U.S., and an analyst said annual sales of the drug could surpass $2 billion a year. The companies plan to launch the drug during this year’s fourth quarter.

Linzess, which was studied under the name linaclotide, has been seen as a major drug for Ironwood and Forest. It is a capsule intended to be taken once per day on an empty stomach, half an hour before a patient’s first meal of the day. While it is approved for adults, the drug will come with a warning label saying it shouldn’t be used in patients age 16 and younger.

Jefferies & Co. analyst Corey Davis said Linzess should become one of Forest’s biggest and most significant products even though it is less profitable than some of the company’s other drugs. Davis estimated that Linzess should cost around $7 per day, and annual sales will peak at more than $2 billion. He added that Linzess probably won’t have significant competition any time soon.

New York-based Forest and Ironwood, of Cambridge, Mass., will market the drug together in the U.S. Spanish drugmaker Almirall SA has filed for marketing approval in Europe, and Astellas Pharma of Japan holds the rights to the drug in Japan and several other Asian countries.

Shares of Ironwood Pharmaceuticals declined 24 cents to $12.42 Thursday, and Forest Laboratories shares rose 38 cents to $35.04.