A drug to treat adult chronic constipation with no known cause won federal approval Tuesday. Lubiprostone works by increasing the secretion of intestinal fluids, which help ease the passage of stool and alleviate symptoms associated with chronic idiopathic constipation, the Food and Drug Administration said.

Chronic constipation is considered idiopathic when it is not caused by other diseases or medications. The condition affects an estimated 3.1 million Americans.

Sucampo Pharmaceuticals Inc., of Bethesda, Md., and Takeda Pharmaceuticals America, of Lincolnshire, Ill., intend to market the drug in capsule form as Amitiza, the FDA said. The drug is the first of its chemical type, it added.

The drug is to be taken twice daily with meals.

Side effects include headache, nausea, diarrhea, abdominal pain and bloating.