The short answer
Keytruda (Pembrolizumab) is a type of immunotherapy called an immune checkpoint inhibitor. It blocks a protein called PD-1 that cancer can use to hide from the immune system, which helps the body's own immune cells find and attack the cancer. It is used to help treat melanoma, non-small cell lung cancer, head and neck cancers, and other cancers, and is usually given as an IV infusion every few weeks. Like all cancer medicines it can cause side effects; this page explains the common ones and the warning signs to report. It is educational only and not a substitute for your care team's advice.
Keytruda is the brand name; its generic name is pembrolizumab.
It is a type of immunotherapy called an immune checkpoint inhibitor — it blocks a protein called PD-1 that cancer can use to hide from the immune system, which helps the body's own immune cells find and attack the cancer.
It is used to help treat melanoma, non-small cell lung cancer, head and neck cancers, and several other cancers.
It is usually given as an IV infusion every few weeks.
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The full explanation.
How blocking PD-1 helps your own T cells
T cells are the immune cells that kill damaged and infected cells. They carry a receptor called PD-1 that acts as an off switch. Healthy tissue uses that switch so the immune system does not attack the body itself.
Many cancers copy the trick. They display the matching protein, PD-L1, press the switch, and the T cell walks away.
Pembrolizumab is an antibody that sits on PD-1 so the cancer cannot reach it. Nothing is added to the T cells. The block is simply removed. Keytruda is the brand name; pembrolizumab is the generic name.
Approved by biomarker, not only by body part
Most cancer drugs are approved for a named organ. Two of the pembrolizumab approvals are not. They are written around a fault in the tumor itself, whatever part of the body it grew in.
One covers solid tumors that are microsatellite instability-high or mismatch repair deficient, when they have grown after earlier treatment and there is no good alternative. A second one, for tumors with a high mutation burden of at least 10 mutations per megabase, works the same way.
So two people with different cancers may be offered the same drug because their tumors share a repair fault. A laboratory test on the tumor, not the organ it grew in, is what decides.
PD-L1 scores and what they decide
For many other uses, a pathologist measures how much PD-L1 the tumor shows. You may see two scoring systems.
TPS, the tumor proportion score, counts the share of cancer cells that stain. CPS, the combined positive score, counts cancer cells plus nearby immune cells against the number of cancer cells.
Different approvals set different cut-offs, such as CPS of 1 or 10. A score below the cut-off does not mean the immune system has failed you. It means this particular drug has not been shown to help enough in that group.
The list of cancers is long
Pembrolizumab is approved across melanoma, lung, head and neck, lymphoma, bladder and urinary tract, bowel, stomach, esophagus, cervix, liver, bile duct, Merkel cell, kidney, womb lining, skin squamous cell, breast and other cancers. Some approvals are for the drug alone; many are only in a set combination.
Being on that list is not the same as being right for you. Ask your team which specific line applies, and what result it was based on.
An infusion, and now a shot
The usual form is a drip into a vein over about 30 minutes, repeated every three weeks or every six weeks.
There is also a version given under the skin, called Keytruda Qlex, which combines pembrolizumab with an enzyme that lets it spread through the tissue. It covers many but not all of the same uses.
Immune-related side effects and steroids
Tiredness, rash, itching, loose stools, nausea and aching joints are the frequent effects.
The ones that matter most come from the immune system attacking healthy tissue. They can appear in any organ. The lungs, bowel, liver, skin, kidneys, and the thyroid, pituitary and adrenal glands are the common targets. Heart muscle inflammation is rare but dangerous.
Treatment is usually steroids, sometimes for weeks, with the dose lowered slowly. That is why reporting early matters so much: a mild problem often needs a short course, while a severe one can end treatment altogether.
These reactions do not follow the infusion calendar. They can begin months in, and they can begin after your last dose.
Symptoms that should not wait
Call your team, day or night, for a new cough or breathlessness, chest pain, or a fast or uneven heartbeat. Call for diarrhea that is watery, bloody, or waking you at night. Call for yellow skin or eyes. Call for a rash that blisters or peels, for severe headache or vision changes, and for extreme tiredness with dizziness or feeling cold all the time.
Tell any other doctor you see, including in an emergency room, that you are on an immunotherapy. It changes how your symptoms should be read.
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Common questions
What is Keytruda?
Keytruda (Pembrolizumab) is a type of immunotherapy called an immune checkpoint inhibitor. It blocks a protein called PD-1 that cancer can use to hide from the immune system, which helps the body's own immune cells find and attack the cancer. It is used to help treat melanoma, non-small cell lung cancer, head and neck cancers, and several other cancers.
How is Keytruda given?
It is usually given as an IV infusion every few weeks. It is given as an intravenous (IV) infusion — a slow drip into a vein — at a clinic or hospital, usually repeated on a schedule of cycles with rest periods in between. The exact schedule is set by your care team.
What are the common side effects of Keytruda?
Commonly reported side effects include tiredness (fatigue), skin rash or itching, diarrhea, nausea, and joint or muscle aches. Not everyone gets them, and many can be managed. Tell your care team about anything new or worsening.
Does Keytruda cure cancer?
That depends on the person, the cancer type, and its stage. For some people a medicine like this can control cancer for a long time or be part of a curative plan; for others the goal is to slow the cancer or ease symptoms. Your care team can explain the goal in your situation.
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Sources last checked: 2026-07-14 what this meansLast updated: 2026-08-09Next planned review: 2027-01-14
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Editorial review complete — This page completed Cancer Explained's editorial checks (sources, safety, plain language, duplication). It has not been reviewed by a physician or other healthcare professional.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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