Ironwood Pharmaceuticals Inc., of Cambridge, and Forest Laboratories Inc. said Monday that Ironwood’s new drug for irritable bowel syndrome with constipation is now available in the United States.

The drug is being marketed in the United States under the brand name Linzess. It was approved by the Food and Drug Administration earlier this year as a treatment for adults suffering from irritable bowel syndrome with constipation. Linzess is a trade name for linaclotide.