The makers of Linzess hit Teva Pharmaceuticals USA with another infringement suit Friday, accusing the Pennsylvania-based generic drugmaker of mounting a late challenge to a patent for the popular constipation drug.

The complaint, filed by Forest Laboratories, Allergan USA Inc. and Ironwood Pharmaceuticals Inc. in U.S. District Court for the District of Delaware, accuses Teva of infringing a 10 patent for Linzess, which is used to treat irritable bowel syndrome with constipation and chronic idiopathic constipation. Last year, the companies sued Teva and a contingent of other pharmaceutical firms for trying to bring their generics to the market.

In Friday’s complaint, the plaintiffs said the 2016 abbreviated new drug application Teva filed with the U.S. Food and Drug Administration identified nine patents as invalid and unenforceable, but made no mention of the so-called ’371 patent at the heart of the new lawsuit.

It wasn’t until Sept. 11, the companies said, that they finally received written notice from Teva regarding the ’371 patent.

“Teva recently amended the Teva ANDA to include, for the first time, allegations that the claims of the ’371 patent are invalid, unenforceable, and/or will not be infringed by the manufacture, use, or sale of the Teva generic products,” they wrote in the complaint, signed by Maryellen Noreika, a partner with Morris, Nichols, Arsht & Tunnell.

“If Teva commercially manufactures, uses, offers for sale, or sells within the United States, or imports into the United States, any of the Teva generic products, or induces or contributes to any such conduct, it would further infringe these claims of the ’371 patent.”

According to the complaint, the generic tablets contain linaclotide, the same active ingredient used in Linzess.

A spokeswoman for Teva did not immediately respond Friday to a call requesting comment on the suit.

Allergan, a Delaware corporation based in New Jersey, is Linzess’ exclusive distributor in the United States. Ironwood owns most of the patents in the suit, and Forest is the exclusive licensee for marketing and selling products containing linaclotide.

All three companies asked the court for monetary and injunctive relief, as well as orders postponing the effective date of the ANDA approvals until after the patents’ last expiration date. They also asked the court to designate the case as exceptional, which would open the door to enhanced damages.

The first case, filed last November, is pending before U.S. District Judge Richard G. Andrews, who has been asked to dismiss the case for improper venue, following the U.S. Supreme Court’s ruling in TC Heartland v. Kraft Foods Group Brands, which tightened rules for establishing venue in patent infringement cases.

The plaintiffs are also represented by Jack B. Blumenfeld, also from Morris Nichols.

The case is captioned Forest Laboratories v. Teva Pharmaceuticals.