Keytruda (pembrolizumab) is an immunotherapy drug used to treat more than a dozen types of cancer. Rather than attacking cancer cells directly like chemotherapy does, it works by helping your immune system recognize and destroy them. It is one of the most widely prescribed cancer drugs in the world, approved for cancers ranging from melanoma and lung cancer to breast, cervical, and liver cancers.
How Keytruda Works
Your immune system has built-in brakes called checkpoints that prevent T cells (a type of white blood cell) from attacking healthy tissue. Cancer cells exploit this system. They display a protein on their surface that binds to a receptor called PD-1 on T cells, essentially telling those cells to stand down. The result: your immune system ignores the tumor.
Keytruda is a checkpoint inhibitor. It attaches to the PD-1 receptor on T cells and blocks cancer cells from flipping that “off switch.” Once the brake is released, T cells can recognize tumor cells as threats and mount an attack against them. This is fundamentally different from chemotherapy, which kills fast-dividing cells indiscriminately. Keytruda reactivates an immune response that was already there but being suppressed.
Cancers Keytruda Is Approved to Treat
Keytruda has FDA approval for an unusually long list of cancers. The major ones include:
- Melanoma (advanced skin cancer)
- Non-small cell lung cancer (the most common type of lung cancer)
- Head and neck cancer
- Bladder and urinary tract cancer
- Esophageal and gastric (stomach) cancer
- Cervical cancer
- Liver cancer
- Kidney cancer
- Triple-negative breast cancer
- Endometrial (uterine) cancer
- Merkel cell carcinoma (a rare, aggressive skin cancer)
- Biliary tract cancer (cancers of the bile ducts and gallbladder)
- Malignant pleural mesothelioma
- Cutaneous squamous cell carcinoma (a common skin cancer)
More recent label updates in 2025 added approvals for certain blood cancers, including classical Hodgkin lymphoma in children and primary mediastinal large B-cell lymphoma in adults and children who haven’t responded to at least two prior treatments.
Tumor-Agnostic Approvals
What makes Keytruda unusual is that two of its approvals aren’t tied to where the cancer started in the body. Instead, they’re based on genetic features of the tumor itself.
The first applies to cancers that are “microsatellite instability-high” (MSI-H) or have deficient mismatch repair (dMMR). These tumors have a defect in their ability to fix DNA errors, which causes them to accumulate many mutations. That high mutation count makes them more visible to the immune system once Keytruda removes the checkpoint block.
The second applies to tumors with a high tumor mutational burden, defined as 10 or more mutations per megabase of DNA. In clinical trials, about 13% of patients tested met this threshold. The logic is the same: more mutations mean more abnormal proteins on the tumor’s surface, giving T cells more targets to attack. Your oncologist will order specific tests on your tumor tissue to determine whether either of these biomarkers applies to you.
How Well It Works
Keytruda’s effectiveness varies significantly depending on the cancer type, how advanced it is, and whether it’s used alone or with other treatments. In advanced melanoma, results from the landmark KEYNOTE-001 trial showed a five-year overall survival rate of 34% across all patients and 41% among those receiving it as their first treatment. Median overall survival was about 24 months for all patients and nearly 39 months for treatment-naive patients. For context, before modern immunotherapy, five-year survival for advanced melanoma was in the single digits.
Keytruda is often combined with chemotherapy, particularly in lung cancer and triple-negative breast cancer, where the combination tends to outperform either approach alone. In some earlier-stage cancers, it’s also used before or after surgery to reduce the risk of the cancer returning.
What Treatment Looks Like
Keytruda is given as an intravenous infusion, typically over about 30 minutes. There are two standard schedules: every three weeks or every six weeks. Both deliver the same overall drug exposure, so the choice often comes down to convenience and what fits your treatment plan. A newer subcutaneous (under-the-skin) injection formulation was also approved, which takes less time to administer.
Treatment duration depends on the situation. For advanced cancers, many patients continue Keytruda for up to two years as long as it’s working and side effects remain manageable. In some cases where the cancer is being treated before or after surgery, the course may be shorter and more defined. Your oncologist will reassess your response through imaging scans at regular intervals throughout treatment.
Side Effects
Because Keytruda works by releasing the brakes on the immune system, it can sometimes cause the immune system to attack healthy tissues. These are called immune-related adverse events, and they’re the most important category of side effects to understand.
Across clinical studies, about 40% of patients experience some type of immune-related side effect. Most are mild to moderate: skin rashes, fatigue, diarrhea, or thyroid problems that cause the gland to become overactive or underactive. Thyroid issues are among the most common and often require ongoing hormone replacement even after treatment ends.
Roughly 20% of patients develop more serious immune-related reactions. These can affect the lungs (causing inflammation that leads to cough and shortness of breath), the liver, kidneys, intestines, or heart. Lung and heart inflammation, while less common, can be dangerous if not caught early. This is why your care team will monitor bloodwork and symptoms closely throughout treatment. Most immune-related side effects are reversible when caught early and treated with steroids or by pausing Keytruda, though some, like thyroid damage, can be permanent.
Notably, Keytruda has no absolute contraindications listed on its label, meaning there is no single condition that categorically prevents its use. However, patients with active autoimmune diseases or organ transplants require careful evaluation, since releasing immune system brakes could worsen autoimmune flares or trigger transplant rejection.
Biomarker Testing Before Treatment
For several of its approved uses, Keytruda is only appropriate if your tumor tests positive for specific biomarkers. In lung cancer, doctors test for PD-L1 expression, which indicates how much of the “off switch” protein the tumor is displaying. Higher PD-L1 levels generally predict a better response to Keytruda, and in some cases, a high PD-L1 score qualifies you for Keytruda alone rather than in combination with chemotherapy.
For the tumor-agnostic indications, testing involves checking whether the tumor is MSI-H/dMMR or has a high mutational burden. These tests are typically done on a biopsy sample and may take a week or more to return results. If your oncologist is considering Keytruda, they will almost certainly order one or more of these biomarker tests as part of the workup. The results don’t just determine eligibility; they also help predict how likely you are to benefit from treatment.