The Food and Drug Administration said Thursday that it has approved a drug candidate from Ironwood Pharmaceuticals Inc. of Cambridge to treat some cases of chronic constipation and irritable bowel syndrome.

Chronic constipation affects an estimated 63 million people, and Ironwood’s drug, called linaclotide, has the potential to be a blockbuster drug, with some stock analysts projecting that the Ironwood drug could ring up sales of more than $1 billion a year worldwide.

Linaclotide is a capsule taken once daily on an empty stomach, and it now can be marketed in the United States to treat chronic idiopathic constipation and irritable bowel syndrome with constipation in adults, the FDA said in a press release. Irritable bowel syndrome is sometimes referred to as IBS.

A recent Globe story noted that Ironwood has been rapidly expanding its 325-person workforce in anticipation of for linaclotide’s launch; it has begun hunting for potential sites for a larger headquarters and research campus in Cambridge, Boston, or the western suburbs.