Lawsuit Claims Dupixent Side Effects Led to CTCL Cancer Diagnosis

California man has undergone chemotherapy, radiation therapy and numerous other treatments to fight Mycosis Fungoides, a form of CTCL.

According to a recently filed lawsuit, John Mun was prescribed Dupixent four years ago to treat atopic dermatitis without being warned of an increased cancer risk, which he only discovered after being diagnosed with a form of T-cell lymphoma, known as Mycosis Fungoides.

The complaint (PDF) was brought in the U.S. District Court for the Southern District of California on January 8, alleging that the defendants, Regeneron Pharmaceuticals Inc., Sanofi-Aventis U.S. LLC and Genzyme Corporation, failed to warn, and likely purposefully hid, the adverse health risks of Dupixent from patients and the medical community.

Dupixent (dupilumab), an injection administered every two to four weeks for the treatment of atopic dermatitis, also known as atopic eczema, was first approved for use by the U.S. Food and Drug Administration (FDA) in 2017. It was later also approved to treat asthma, chronic obstructive pulmonary disease (COPD) and other inflammatory diseases affecting the respiratory tract and skin, with more than 1 million patients receiving the injection in recent years.

However, while the drug has been marketed as safe and effective, concerns quickly arose after approval about a potential link between Dupixent and T-cell lymphoma, a rare form of cancer that impacts the body’s white blood cells, which are a key part of the immune system. These concerns have linked Dupixent side effects with an increased risk of cutaneous T-cell lymphoma (CTCL) and peripheral T-cell lymphoma (PTCL).

Mun’s claims are the latest to join a growing number of Dupixent cancer lawsuits filed throughout the U.S. in the last couple months, each raising similar claims that the manufacturers knew, or should have known, that Dupixent side effects increased CTCL and PTCL risks. In each of the claims, plaintiffs allege the manufacturers placed profits ahead of consumer safety by keeping knowledge of those risks suppressed.

The California man was first prescribed Dupixent in May 2022, and continued to use the drug until February 2023, according to the complaint. That February he was diagnosed with Mycosis Fungoides, which is a form of CTCL.

Following the diagnosis, to preserve his health and quality of life, Mun has undergone a wide range of extensive medical treatments, including chemotherapy, radiation phototherapy, topical steroids, cancer inhibitors and various other medications.

“Plaintiff, John Mun has been advised by his healthcare providers that he will likely require ongoing treatments and healthcare services for CTCL indefinitely. Since his initial diagnosis of CTCL in 2023, Plaintiff, John Mun, has continued to suffer from new tumor growth.”

– John Mun v. Regeneron Pharmaceuticals Inc. et al

The lawsuit alleges there was enough evidence before Mun was prescribed the drug for the manufacturers to have learned of potential Dupixent side effects increasing the risk of cancer, and to have updated the labels. However, Mun indicates that he did not learn the manufacturers knew about the risks until the FDA posted data on a potential safety signal in March 2025.

Even then, the FDA had first warned Regeneron about the safety issues in January 2025, before issuing the data publicly several months later. The lawsuit notes that, currently, the manufacturers have still failed to update the label to warn of Dupixent cancer side effects.

Mun presents claims of failure to warn, negligence, fraudulent misrepresentation and negligent misrepresentation. He seeks both compensatory and punitive damages.

Dupixent CTCL Lawsuits

The complaint joins a growing number of Dupixent lawsuits raising similar allegations that the medication may be linked to diagnoses of CTCL, PTCL or other types of T-cell lymphoma.

Patients who used Dupixent and were later diagnosed with one of these illnesses may be eligible to pursue legal claims seeking compensation for their injuries and related losses.

Dupixent lawyers provide no-cost consultations to review medical records, discuss possible claims and determine whether individuals may be eligible for a Dupixent lawsuit settlement. 

These claims are handled on a contingency basis, meaning there are no upfront legal costs and attorney fees are only paid if a settlement or verdict is obtained.

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