Keytruda (pembrolizumab) is an immunotherapy drug used to treat more than 20 types of cancer, making it one of the most widely prescribed cancer treatments in the world. Rather than attacking cancer cells directly like chemotherapy does, Keytruda works by helping your immune system recognize and destroy them. It’s approved for cancers ranging from lung cancer and melanoma to bladder, kidney, and cervical cancers, and new uses continue to be added.
How Keytruda Works
Cancer cells survive partly by hiding from the immune system. They do this by displaying a protein called PD-L1 on their surface, which acts like a “don’t attack me” signal. When PD-L1 connects with a receptor called PD-1 on immune cells (T cells), it essentially switches those T cells off. The T cells become exhausted and dysfunctional, unable to mount an attack even though they’re sitting right next to the tumor.
Keytruda is an antibody that blocks PD-1 on T cells, preventing cancer cells from flipping that off switch. With the brake released, T cells can recognize the tumor as a threat and begin killing cancer cells. This is why Keytruda is classified as an “immune checkpoint inhibitor.” It doesn’t create a new immune response; it removes the block that was preventing one.
Cancers Keytruda Treats
Keytruda has one of the broadest approval lists of any cancer drug. Its major uses fall into several categories.
Lung Cancer
Non-small cell lung cancer (NSCLC) is the most common reason Keytruda is prescribed. It can be used as a first-line treatment for advanced NSCLC, either alone (when tumors have high PD-L1 levels) or combined with chemotherapy. In a five-year study of patients with high PD-L1 expression who received Keytruda as their first treatment, the median overall survival was 19.1 months, and about 25% of patients were still alive at five years. Patients with fewer sites of spread and non-squamous tumors tended to do better.
Melanoma
Keytruda was originally approved for advanced melanoma in 2014, and it remains a cornerstone treatment. It’s used for melanoma that can’t be surgically removed or has spread, and also after surgery to reduce the risk of recurrence in earlier-stage disease.
Other Solid Tumors
Approved uses span a long list: bladder cancer (urothelial carcinoma), kidney cancer, head and neck squamous cell cancer, cervical cancer, liver cancer, stomach cancer, esophageal cancer, and endometrial cancer, among others. In February 2026, the FDA approved Keytruda in combination with chemotherapy for platinum-resistant ovarian, fallopian tube, or primary peritoneal cancer when tumors express PD-L1.
Cancers With Specific Genetic Markers
One of Keytruda’s most notable approvals is “tumor-agnostic,” meaning it’s based on the biology of the cancer rather than where it started. Tumors that are microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) can be treated with Keytruda regardless of their location in the body. These genetic features mean the cancer has many mutations, which tends to make it more visible to the immune system once the PD-1 brake is removed. Tumors with a high tumor mutational burden (TMB-H) also qualify.
Biomarker Testing Before Treatment
Not everyone with a qualifying cancer type is automatically a candidate for Keytruda. For many indications, your oncologist will order tests on your tumor tissue first. The most common test measures PD-L1 expression, reported as a score that reflects how much of the protein your tumor produces. Higher PD-L1 levels generally predict a better response, and some approvals require a minimum score for eligibility.
For the tumor-agnostic approvals, testing focuses on MSI-H or dMMR status, which can be assessed through a biopsy sample. These tests help determine whether your particular cancer is likely to respond to immunotherapy, which is important because Keytruda doesn’t work for everyone, and unnecessary treatment means unnecessary side effects.
Alone or With Other Treatments
Depending on the cancer type and stage, Keytruda may be given as a single drug or combined with chemotherapy or other targeted therapies. In advanced lung cancer, for example, combining Keytruda with chemotherapy improves both progression-free survival and overall survival compared to either approach alone. The combination approach is standard for many patients whose tumors don’t have very high PD-L1 levels.
For melanoma and certain other cancers, Keytruda is often effective on its own. Your oncologist determines the approach based on your cancer type, biomarker results, and overall health.
What Treatment Looks Like
Keytruda is given as an intravenous infusion, typically over about 30 minutes. There are two approved schedules for adults: 200 mg every three weeks or 400 mg every six weeks. Both deliver the same total drug exposure over time, so the six-week option, approved by the FDA in 2020, simply means fewer trips to the infusion center. Treatment continues as long as it’s working and side effects remain manageable, often up to two years.
Common Side Effects
Because Keytruda activates the immune system, its side effects look different from those of traditional chemotherapy. You’re less likely to experience severe nausea or hair loss, but more likely to develop inflammation-related problems. The most common short-term effects are skin rash or itching, thyroid inflammation or low thyroid hormone levels (hypothyroidism), fatigue, and joint pain.
These aren’t just temporary nuisances for everyone. Long-term data show that certain side effects tend to persist even after treatment ends. Joint pain, thyroid problems, adrenal insufficiency, eye issues like blurred vision, and inflammation of the pituitary gland are among the effects most likely to become chronic. Hypothyroidism, for instance, often requires lifelong thyroid hormone replacement.
Serious Immune-Related Reactions
The same immune activation that fights cancer can occasionally turn against healthy tissue. These immune-related adverse events can affect virtually any organ: the lungs (pneumonitis), colon (colitis), liver (hepatitis), kidneys, heart, or nervous system. Most are manageable when caught early and treated with immunosuppressive medications like steroids, but some can be severe or life-threatening. Inflammation of the heart muscle (myocarditis) and neurological conditions resembling Guillain-Barré syndrome are rare but serious examples.
This is why infusion centers monitor you closely during treatment and why you’ll be asked to report new symptoms promptly. Catching inflammation early, before it causes significant organ damage, makes a meaningful difference in outcomes. Most patients tolerate Keytruda well enough to complete their planned course, but about 10 to 20 percent of patients across trials need to stop treatment due to immune-related side effects.
Who Responds Best
Response to Keytruda varies considerably. In lung cancer, five-year data show that roughly one in four patients with high PD-L1 expression are alive five years after starting treatment, a meaningful improvement over what chemotherapy alone historically offered. But the flip side is that a majority of patients eventually see their cancer progress. Patients with fewer than three sites of metastasis had substantially better long-term survival than those with more widespread disease.
Tumor type matters too. Squamous cell lung cancers carried a higher risk of progression than non-squamous types in long-term analysis. Cancers with MSI-H or high tumor mutational burden tend to respond particularly well to checkpoint inhibitors as a group, sometimes producing durable remissions lasting years.