FDA Flags Possible Colitis Risk With CDK4/6 Breast Cancer Drugs

On Sept. 30, 2026, the U.S. Food and Drug Administration (FDA) posted its latest quarterly list of potential safety signals, and several widely used cancer treatments are on it. The list names colitis (inflammation of the large intestine) as a possible risk with three breast cancer pills — Ibrance (palbociclib), Kisqali (ribociclib) and Verzenio (abemaciclib) — and myocarditis (inflammation of the heart muscle) as a possible risk with a group of targeted drugs used for melanoma and other cancers. For each one, the FDA says it "is evaluating the need for regulatory action."

A listing is an early warning, not a final finding. It does not mean the FDA has concluded that a drug causes the problem, and it does not mean patients should stop treatment.

What Is the FDA Safety Signal List?

The FDA reviews reports of side effects that doctors, patients and drug companies send to its adverse event reporting system. Each quarter, it publishes drugs for which it has spotted a potential new serious risk or new safety information. This list covers reports the agency identified from April through June 2026. After review, the FDA may decide a label change is needed, decide more study is needed or find no action is required.

Which Cancer Drugs Are on the List

• CDK4/6 inhibitors — possible colitis: Ibrance, Kisqali (including the Kisqali Femara Co-Pack) and Verzenio. These pills are taken by mouth, usually with hormone therapy, for hormone receptor-positive, HER2-negative breast cancer.
• BRAF inhibitors — possible myocarditis: Braftovi (encorafenib), Ojemda (tovorafenib), Tafinlar (dabrafenib) and Zelboraf (vemurafenib). These target tumors with a BRAF gene change and are used for melanoma and certain other cancers.
• MEK inhibitors — possible myocarditis: Cotellic (cobimetinib), Mekinist (trametinib) and Mektovi (binimetinib), which are often given together with a BRAF inhibitor.
• CAR-T cell therapies — possible immune effector cell-associated enterocolitis (inflammation of the intestines linked to immune cell treatment): Abecma (idecabtagene vicleucel), used for multiple myeloma, and Breyanzi (lisocabtagene maraleucel), used for certain lymphomas and leukemias.
• Radioligand therapies — wrong drug given: Lutathera (lutetium Lu 177 dotatate), used for certain neuroendocrine tumors, and Pluvicto (lutetium Lu 177 vipivotide tetraxetan), used for certain prostate cancers. This entry concerns medication errors in which one treatment was given in place of the other, not a side effect of either drug.

What This Means for You

If you take one of these drugs, keep taking it as prescribed unless your care team tells you otherwise. These medicines are approved because studies showed their benefits outweigh their known risks for the people they treat. The FDA has not changed any of these labels based on this list.

Diarrhea is already a known, common side effect of CDK4/6 inhibitors, especially Verzenio. Colitis is different: it is inflammation of the bowel itself. Tell your care team right away if you have diarrhea that is severe or does not improve with the medicines they recommended, belly pain or cramping, blood or mucus in your stool, or a fever.

For people taking BRAF or MEK inhibitors, call your care team promptly for chest pain or pressure, shortness of breath, a racing or irregular heartbeat, swelling in your legs, or unusual tiredness. Many patients on these drugs already have heart tests during treatment. Learn more about how CDK4/6 inhibitors such as Verzenio are used in advanced breast cancer.

What's Still Unknown

The FDA has not said how many reports it received, how serious they were or whether the drugs caused the problems. Side-effect reports can come from people who also have other illnesses or take other medicines, so a signal needs further review. The FDA will post an update if it takes action, such as adding a warning to a drug label.

References

1. "April - June 2026 | Potential Signals of Serious Risks/New Safety Information Identified by the FDA Adverse Event Reporting System (FAERS)." U.S. Food and Drug Administration. Sept. 30, 2026.