Pembrolizumab

Pembrolizumab is a type of cancer immunotherapy. It helps your immune system find and attack cancer cells. You get it as an IV infusion at a clinic or hospital. This guide explains what it is used for, how it works, key safety warnings, and common side effects to watch for.

Brand Name(s)

Keytruda

Warning(s)

Pembrolizumab can cause your immune system to attack normal organs. These problems can happen during treatment or months after it ends. Tell your care team right away if you notice new or worsening symptoms.

Lungs (pneumonitis): New or worse cough, chest pain, shortness of breath.
Intestines (colitis): Diarrhea, more bowel movements, blood or mucus in stool, severe belly pain.
Liver (hepatitis): Yellow skin or eyes, dark urine, severe nausea or vomiting, right-side belly pain, easy bruising.
Hormone glands (endocrine): Thyroid, pituitary, adrenal, or pancreas problems. Signs include extreme tiredness, headache, dizziness or fainting, mood changes, weight change, feeling very hot or cold, hair loss, constipation or diarrhea, fast heartbeat, increased thirst or urination.
Kidneys (nephritis): Swelling in legs, less urine, blood in urine, weight gain from fluid.
Nerves, muscles, and eyes: Weakness, numbness, tingling, severe muscle pain, double or blurry vision, eye pain, drooping eyelids.
Skin: Rash, itch, blisters, peeling skin, painful sores in mouth or on skin.
Heart and other organs: Chest pain, shortness of breath, fast or irregular heartbeat, swelling, dizziness.
Infusion reactions: Fever, chills, flushing, rash, itching, wheezing, dizziness, or trouble breathing during or soon after the infusion. Tell staff right away.
Transplant risks: If you have had an allogeneic stem cell transplant, serious complications like graft-versus-host disease (GVHD) can occur. If you have an organ transplant, there is a risk of rejection. Your team will weigh these risks.
Pregnancy and breastfeeding: Pembrolizumab can harm an unborn baby. Use birth control during treatment and for a period after your last dose. Do not breastfeed during treatment and for a period after; ask your team when it is safe.

Before starting, make sure your care team knows if you:

Have or had autoimmune disease, lung, liver, kidney, heart, nerve, or hormone gland problems.
Have had a stem cell or organ transplant.
Have any infections.
Are pregnant, plan to become pregnant, or are breastfeeding.
Take any medicines, vitamins, or herbal products (especially steroids or medicines that affect the immune system).
Plan to get vaccines. Avoid live vaccines during treatment and until your care team says it is okay.

Kind of Cancers It Is Used For (not a complete list)

Melanoma (skin cancer), including advanced disease and certain cases after surgery.
Non-small cell lung cancer (NSCLC) and some cases of small cell lung cancer (SCLC).
Head and neck squamous cell carcinoma (HNSCC).
Esophageal and gastric/gastroesophageal junction (GEJ) cancers.
Colorectal cancer that is MSI-H or dMMR, and certain other MSI-H/dMMR solid tumors regardless of where they started.
Tumor mutational burden-high (TMB-H) solid tumors after other treatments.
Urothelial (bladder and urinary tract) cancer.
Cervical cancer.
Endometrial (uterine) cancer, especially dMMR disease.
Hepatocellular carcinoma (liver cancer).
Renal cell carcinoma (kidney cancer), often with other medicines.
Triple-negative breast cancer (TNBC) in certain early-stage high-risk or PD-L1–positive cases.
Cutaneous squamous cell carcinoma (cSCC) in some advanced cases.
Classical Hodgkin lymphoma (cHL) and primary mediastinal large B-cell lymphoma (PMBCL).

Your exact use depends on your cancer type, features like PD-L1, MSI/dMMR, or TMB status, and prior treatments. Your oncology team will test your tumor to guide whether pembrolizumab is appropriate.

Mechanism of Action

Your immune system uses “checkpoints” to avoid attacking healthy cells. Some cancers hide by turning these checkpoints on. Pembrolizumab blocks one of these checkpoints.

It is a PD-1 inhibitor. PD-1 is a protein on immune cells called T cells.
Many tumors show PD-L1, which binds PD-1 and tells T cells to stand down.
By blocking PD-1, pembrolizumab frees T cells to recognize and attack cancer cells.
This can shrink tumors and help them stay under control. It may also cause the immune system to react in healthy organs, which is why monitoring is important.

Common Side Effects

Not everyone has the same side effects. Many are mild to moderate and can be managed. Report symptoms early so your team can help.

Tiredness or weakness
Rash or itching
Diarrhea or constipation
Nausea, vomiting, or decreased appetite
Fever or chills
Headache
Joint or muscle pain
Cough or shortness of breath

Sometimes side effects are more serious and need quick care. Call your care team right away if you notice:

New or worse cough, chest pain, or trouble breathing
Severe diarrhea, black or bloody stools, or bad belly pain
Yellow skin or eyes, dark urine, severe nausea or vomiting, or unusual bleeding
Extreme tiredness, dizziness, fainting, bad headaches, mood changes, or fast heartbeat
Increased thirst or urination, sudden weight changes, or new confusion
Less urine, blood in urine, or swelling in legs or feet
Severe or spreading rash, blisters, or skin peeling; painful mouth sores
Muscle weakness, numbness, tingling, vision changes, or eye pain
Chest pain, fast or irregular heartbeat, or swelling
Fever, chills, flushing, rash, itching, wheezing, or dizziness during the infusion

Tips to support your safety:

Keep all blood tests and scan appointments. They help spot side effects early.
Carry an updated medication list. Include over-the-counter drugs and supplements.
Ask your team before starting new medicines or vaccines.
Use reliable birth control and discuss family planning with your care team.

For more information, see: MedlinePlus: Pembrolizumab

Last reviewed: 2025-10-23