Keytruda: What to Expect, Side Effects, and More

Keytruda (chemical name: pembrolizumab) is a type of immunotherapy that helps your immune system detect and fight cancer cells. 

You may be prescribed Keytruda if:

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  You were diagnosed with early-stage triple-negative breast cancer with a high risk of recurrence (the cancer coming back).

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  You were diagnosed with triple-negative, PD-L1-positive breast cancer that is metastatic or locally advanced (spread to other breast areas) and can’t be removed with surgery.

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  You were diagnosed with metastatic or inoperable breast cancer that has a biomarker called microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) that grew following the previous treatment; MSI-H and dMMR tumors have abnormalities that affect how DNA is repaired.

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  You were diagnosed with metastatic or inoperable breast cancer with tumor mutational burden-high (TMB-H) that grew following the previous treatment; TMB-H means the cancer tumor has a high level of mutations in its DNA.

As of September 2025, there are two forms of Keytruda. The original version is given as an IV infusion. A newer version, Keytruda Qlex, is given as an injection under the skin.

See Keytruda prescribing information.

Keytruda is a type of immunotherapy called an immune checkpoint inhibitor. Immune checkpoints are proteins in your body that help your immune system tell the difference between your cells and foreign invaders, such as harmful bacteria, viruses, or cancer cells. Cancer cells sometimes find ways to use these immune checkpoints as a shield to avoid being identified and attacked by the immune system.

Immune checkpoint inhibitors target these immune checkpoints and help the immune system recognize and attack cancer cells. 

PD-1 is a type of immune checkpoint found on T cells. (T cells are immune system cells that roam throughout the body looking for signs of disease or infection.) PD-L1 is another immune checkpoint found in many healthy cells in the body. When PD-1 binds to PD-L1, it stops T cells from killing a cell.

Some cancer cells have a lot of PD-L1 on their surface. Keytruda stops PD-1 from binding to PD-L1. This allows T cells to attack the cancer cells.

The IV form of Keytruda is given as a 30-minute infusion, every three or six weeks, depending on the dose given at each infusion. If you’re prescribed Keytruda with chemotherapy, the Keytruda infusion is given before the chemotherapy infusion.

The Qlex form of Keytruda is given as an injection into the thigh or abdomen — avoiding the 2-inch area around the navel — in one minute every three weeks, or in two minutes every six weeks.

If you’re receiving Keytruda for early-stage triple-negative breast cancer and your doctor recommends surgery, Keytruda and chemotherapy may first be used to help shrink the cancer. After surgery, your doctor may prescribe Keytruda alone to keep the cancer from coming back.

If you’re receiving Keytruda for metastatic or locally advanced triple-negative breast cancer that can’t be removed with surgery, you may continue to receive Keytruda even after chemotherapy treatment has stopped.

People who are pregnant or planning to get pregnant should not receive Keytruda. Keytruda can cause embryo death and birth defects. If there’s any chance you could become pregnant while receiving Keytruda, you must use birth control.

Like most cancer treatments, Keytruda can cause side effects, some of them severe.

The most common side effects of Keytruda are:

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  fatigue

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  nausea

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  diarrhea

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  constipation

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  vomiting

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  hair loss

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  rash

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  cough

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  loss of appetite

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  headache

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  low white blood cell counts

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  anemia

Keytruda can also cause other serious side effects, including:

Hormone gland problems: Keytruda may affect glands that make hormones your body needs to function properly, including the thyroid, adrenal glands, pancreas, and pituitary gland. Symptoms include headaches that won’t go away, extreme tiredness, weight gain or loss, changes in mood or behavior, feeling cold, and constipation.

Lung problems: Keytruda may cause inflammation of the lungs, which can be life-threatening. Symptoms include trouble breathing, worsening cough, and chest pain.

Liver problems: Keytruda may cause hepatitis (inflammation of the liver). Tell your doctor right away if you have yellowing of your skin or the whites of your eyes, dark-colored urine, bleeding or bruising more easily than normal, severe nausea or vomiting, or pain on the right side of your stomach area.

Colitis (inflammation of the inner lining of the colon): Symptoms include diarrhea, blood or mucus in your stool, and severe stomach pain.

Other organ problems: Keytruda also may affect other organs in your body. Symptoms include severe muscle weakness, confusion, blurry or double vision, neck stiffness, skin blisters, irregular heartbeat, shortness of breath, or swelling of the ankles.

Severe infection: Symptoms include fever, cough, flu-like symptoms, pain when urinating, or back pain.

A 2025 study that collected patient-reported outcomes from of people receiving Keytruda found that rates of immune system-related side effects were similar to those reported in the KEYNOTE-522 trial. The KEYNOTE-522 trial is the study that led to Keytruda’s approval for early-stage triple-negative breast cancer. But the most commonly affected systems were different. Endocrine system side effects were the most frequently reported and some required people to be on lifelong hormonal treatment. Gastrointestinal (GI) system side effects also were more common than reported in the KEYNOTE-522 study. Some GI side effects were so severe that people had to stop receiving Keytruda while other people needed medicine to suppress their immune systems.

Members of our community discuss their experiences with Keytruda in the community forum. Here are some of their comments. Remember, it's very important to talk to your doctor about any side effects you're having and ask about ways to manage them.

“[I had the] usual side effects: fatigue, loss of hair, slight nausea and digestive discomfort/problems.” —nume

“I finished chemo in early July and have since had 4/9 of my stand-alone infusions of Keytruda. I will say that I haven't had any notable symptoms with these infusions, except for some skin redness.” —meghann422

“I started it about nine months ago. [It’s] easy to take — 30 minute IV. No obvious side effects.” —Bosco19

Read more in our Keytruda discussion forum.

The cost of Keytruda may vary. The price you’ll pay depends on the drug manufacturer, your health insurance provider and plan, where you live, and the pharmacy you use.

Note: Your insurance company may require you to get prior authorization before covering Keytruda. 

Merck, the company that makes Keytruda, has a patient support program that may be able to help you. You can call the Merck Access Program at 855-257-3932.

Learn more about financial assistance and medicine cost-lowering tips.