The drug, linaclotide, will be sold under the brand name Linzess and carry a boxed warning that it should not be used in patients 16 or younger, the agency said.

An estimated 63 million people suffer from chronic constipation, according to the National Institutes of Health. Additionally, an estimated 15.3 million people cope with irritable bowel syndrome (IBS), which causes abdominal pain when accompanied by constipation.

Ironwood and marketing partner Forest Laboratories said they expect the drug to be available for patients in the fourth quarter.

Gregory Wade, an analyst with Wedbush Securities, projects Linzess to be a future blockbuster with peak sales exceeding $2 billion by 2019. However, he was estimating sales of just $40 million in 2013, ramping up to $165 million in 2014.

Linzess is a capsule meant to be taken once daily on an empty stomach, at least 30 minutes before the first meal of the day, the FDA said. It relieves constipation by helping bowel movements occur more often and may ease the stomach pain associated with IBS.

Shares of Ironwood Pharmaceuticals, which were halted prior to the approval announcement and for most of the day, were down 3 percent to $12.28 on Nasdaq.

Wade suggested that investors, cautious about the launch of new drugs, may want to see how sales take off before betting on Ironwood.

"No one medication works for all patients suffering from these gastrointestinal disorders," Victoria Kusiak, a deputy director at the FDA's Center or Drug Evaluation and Research, said in a statement. "With the availability of new therapies, patients and their doctors can select the most appropriate treatment for their condition."

Shares of Forest, which is betting on a raft of new drugs to offset declining sales of its flagship antidepressant Lexapro because of generic competition, were up 1 percent at $35.01 on the New York Stock Exchange.

(Reporting by Bill Berkrot; Editing by Gerald E. McCormick and M.D. Golan)

• A Permanent Economic Emergency