The US Food and Drug Administration moved Wednesday to require new warnings about colitis on the labels of three widely used breast cancer drugs — Pfizer's Ibrance, Novartis's Kisqali and Eli Lilly's Verzenio — after completing a safety evaluation of the CDK4/6 inhibitor class. The agency identified cases of serious gastrointestinal inflammation in patients taking the medicines.

The three drugs belong to a class known as CDK4/6 inhibitors, prescribed to hundreds of thousands of patients worldwide for hormone receptor-positive, HER2-negative breast cancer. The FDA had flagged reports of colitis for further scrutiny before deciding on label action.

The FDA said clinicians should monitor patients for persistent diarrhoea, abdominal pain and blood in the stool. In confirmed cases, doctors should consider interrupting or discontinuing treatment. An FDA spokesperson said the benefits of the drugs continued to outweigh their risks for approved uses, but that prescribers needed clearer guidance on recognising and managing the condition.

Pfizer, Novartis and Eli Lilly each said they were working with the FDA to update prescribing information and would notify oncologists. The companies emphasised that colitis remained a rare adverse event and urged patients not to stop treatment without consulting their physicians.

CDK4/6 inhibitors have become a cornerstone of advanced breast cancer care since Ibrance's 2015 approval, with the class generating billions of dollars in annual sales. Oncology groups, including patient advocates at Susan G. Komen, have called for stronger post-market monitoring as the drugs are used earlier in treatment and for longer durations.