A Keytruda lawsuit is a product liability claim alleging that Merck failed to adequately warn patients and doctors that its cancer immunotherapy can cause serious heart damage, kidney injury, and other immune-mediated harm. The suits target injuries such as myocarditis, cardiomyopathy, and acute kidney injury, and they proceed on failure-to-warn and negligence theories. Litigation is in relatively early stages, and filing deadlines run from the state where the patient was treated.1
Injuries the Lawsuits Cover
The product liability cases center on three injuries: myocarditis (inflammation of the heart muscle), cardiomyopathy (weakening of the heart), and acute kidney injury. A wrongful death suit filed in Arkansas on September 24, 2025 is representative: the plaintiff alleged that Keytruda caused a family member to develop fatal acute cardiomyopathy, acute kidney injury, and myocarditis, and brought claims for strict products liability based on failure to warn, as well as negligence.
Other immune-mediated reactions listed on the drug’s own label — pneumonitis, colitis, hepatitis, endocrine disorders (including checkpoint inhibitor-induced type 1 diabetes), nephritis, and severe skin reactions — sit in the same family of injuries. They stem from the way the drug works: Keytruda blocks the PD-1 receptor that cancer cells use to hide from the immune system, and that same broad immune activation can turn the body against its own tissues.
What Plaintiffs Say Merck Knew
The core allegation is that Merck made what one complaint called a “profit-driven decision” to withhold safety information. Plaintiffs contend Merck held internal data from studies spanning 2016 through 2019 showing elevated risks of heart damage and kidney injury but did not update the label to match. The suits allege the current warnings understate how often these injuries occurred in clinical trials, including cases that ended in death, and fail to flag that certain groups face greater danger: elderly patients, and patients with pre-existing kidney disease or autoimmune conditions.
Plaintiffs also allege that Merck aggressively marketed Keytruda to elderly patients while knowing this group was especially vulnerable to serious harm.
The Legal Theories
Two theories carry most of these cases.
Strict products liability — failure to warn. A drug manufacturer can be held liable for selling a product without adequate warnings about known risks, regardless of whether the company was otherwise careful. If the warnings on the label do not match what the company’s own data showed, that alone can establish liability.
Negligence. This requires showing Merck fell below the standard of care a reasonable pharmaceutical company would have followed, for example by not updating the label when internal data warranted an update.
Liability can potentially reach beyond Merck. Legal commentators note that prescribing physicians who failed to relay known risks, hospitals that inadequately monitored patients during infusions, and clinical trial sponsors could face claims in some circumstances.
The Medical Evidence Behind the Claims
The medical literature gives the lawsuits a factual spine. A pooled analysis of nearly 9,000 patients across 31 clinical trials of Keytruda monotherapy found that about 24% experienced at least one immune-mediated adverse event or infusion reaction, and roughly 6% experienced serious ones. Twenty-two patients, 0.2% of the study population, died from immune-mediated reactions. Pneumonitis was the leading cause of death (15 cases), followed by colitis and myocarditis (2 each).
Myocarditis is rare but especially deadly. One review of immune checkpoint inhibitor data found a mortality rate of nearly 40% for myocarditis, making it the deadliest immune-mediated side effect on a per-case basis. A case report in JACC: Case Reports documented a fatal case of pembrolizumab-induced myocarditis in which the patient’s left ventricular ejection fraction dropped to 20–25%.
Checkpoint inhibitor-induced type 1 diabetes affects roughly 1–2% of patients, presents with rapid-onset diabetic ketoacidosis, and is typically permanent. Nearly 90% of affected patients require ICU care and become insulin-dependent for life. Unlike some other immune-mediated reactions, this form of diabetes does not respond to immunosuppressive treatment.
FDA Actions That Back Up the Timeline
Federal regulators have taken several actions that narrowed Keytruda’s approved uses or tightened its use, giving plaintiffs corroboration for the argument that risks were understated for too long.
In June 2018, the FDA restricted Keytruda as a first-line treatment for advanced bladder cancer in patients who cannot receive cisplatin chemotherapy, limiting it to patients whose tumors express high levels of PD-L1. The change followed data monitoring findings from the KEYNOTE-361 trial that patients with low PD-L1 expression who received Keytruda alone had decreased survival compared to those who received chemotherapy.
In September 2024, the FDA’s Oncologic Drugs Advisory Committee voted 11-to-1 that there was insufficient evidence to support Keytruda’s use in stomach cancer patients with low PD-L1 expression, concluding risks outweighed benefits for that group. By June 2025, the FDA had formally limited Keytruda’s gastric cancer indications to patients meeting minimum PD-L1 thresholds.
In August 2024, Merck itself stopped two Phase 3 trials on recommendations from independent data monitoring committees. KEYNOTE-867 studied Keytruda combined with radiation for early-stage lung cancer; the combination failed to improve survival and was associated with higher rates of adverse events, including deaths, compared to placebo. KEYNOTE-630 studied Keytruda as a follow-up treatment for high-risk skin cancer.
Deadlines to File
Statutes of limitations vary by state. In California, for example, the deadline is two years from the date a patient becomes aware that Keytruda may have caused the injury. That “discovery” trigger matters: for injuries like myocarditis or acute kidney injury that appear during or shortly after treatment, the clock can start well before a patient connects the dots to the drug. Anyone considering a claim should confirm the applicable deadline for the state where treatment occurred before assuming time remains.
The product liability litigation is still developing, with attorneys evaluating cases from patients who developed myocarditis, cardiomyopathy, or acute kidney injury after Keytruda treatment.
Patent Cases Are a Separate Track
Keytruda is also the subject of major patent litigation, including a $625 million settlement Merck paid to Bristol-Myers Squibb and Ono Pharmaceutical in 2017 and an ongoing fight with Halozyme Therapeutics over the newer subcutaneous formulation, Keytruda QLEX. Those disputes are between companies over intellectual property and do not involve patient injuries. A search that turns up “Keytruda lawsuit” headlines about patents is looking at a different track from the product liability claims described above.