ALLEGAN, Mich. Perrigo has filed for regulatory approval of a generic version of an over-the-counter medication for frequent heartburn, prompting a lawsuit from the branded version’s manufacturer.

The company announced Friday that it had filed for approval for omeprazole and sodium bicarbonate in the 20 mg/1,100 mg strength. The medication is a generic version of Zegerid OTC, made by Schering-Plough HealthCare, a subsidiary of Merck.

Schering-Plough filed a lawsuit Monday alleging patent infringement in the U.S. District Court for the District of New Jersey to prevent Perrigo’s commercialization of the product.

Zegerid had sales of around $60 million during the 12-month period ended in the “most recent month,” according to SymphonyIRI Group.