Keytruda (pembrolizumab)

Immunotherapy (PD-1 blocker) · Used for: Lung cancer, Breast cancer, Colorectal cancer

💉 Given by IV or injection · Immunotherapy (PD-1 blocker)

Cliff notes

What it is: Immunotherapy (PD-1 blocker).

How it's given: An IV infusion, usually over about 30 minutes, every 3 or 6 weeks.

How it works: Keytruda is an immune checkpoint inhibitor. It blocks a protein called PD-1 on your immune cells. Cancer cells can use PD-1 to hide. When it is blocked, your immune system can find and attack the cancer better. Because it boosts your immune system, it can also make the immune system attack healthy organs.

Common side effects and what helps at home

Side effect	What can help
Tiredness (fatigue)	Keep a gentle routine, take short walks and rest when you need to.
Rash and itchy skin	Use gentle, fragrance-free soap and moisturizer. Tell your team about any new rash.
Diarrhea	Drink plenty of fluids. Call your team early, because with immunotherapy diarrhea can be a sign of colon inflammation.
Fever, itching, cough and constipation	These are also among the most common side effects in the FDA label. Report a new cough or fever.
Muscle, bone or joint pain	Ask your team which pain relievers are OK for you.
Decreased appetite, nausea	Try small meals often.
Cough or shortness of breath	Always report these. They can be a sign of lung inflammation.
Underactive thyroid (hypothyroidism)	Very common. Your team checks thyroid blood tests; tell them about tiredness, feeling cold or weight gain.

Not everyone gets these. Your team may suggest other ways to manage them.

Call your care team now if you have:

New or worse cough, chest pain or shortness of breath
Diarrhea, many more bowel movements than usual, or black, tarry or bloody stools
Yellow skin or eyes, dark urine, or pain on the right side of your belly
Severe tiredness, headaches that won't go away, dizziness or fainting (possible hormone gland problems)
Peeing much less, or blood in your urine
Rash with blistering, peeling or mouth sores
Fever of 100.4°F (38°C) or higher
During or after the infusion: chills, shaking, itching, flushing, trouble breathing or feeling faint (infusion reaction)
Immune side effects can start any time, even months after your last dose, so always mention you've had Keytruda

Call 911 for trouble breathing, chest pain or signs of stroke.

🗓 What to expect

1️⃣ Before you start

Ask your team for a written schedule and an after-hours phone number.
Bring a list of all your medicines and supplements.

2️⃣ During each infusion

You'll get the medicine through an IV in a clinic; nurses check you during the infusion.
Tell the nurse right away about chills, itching, flushing or trouble breathing.
Bring a snack, water and something to pass the time.

3️⃣ Between doses

Watch for the side effects below and use the checklist.
Keep up with blood tests and visits.
Call your team for anything in the red box.

📅 Sample 21-day cycle

= 💉 infusion. Every 3 weeks is one option; every 6 weeks is another. This is an example; follow the schedule your team gives you.

✅ Side-effect checklist

Print this and check off what you notice so you can tell your team.

Side effect	Noticed?	Notes for my team
Tiredness (fatigue)	☐
Rash and itchy skin	☐
Diarrhea	☐
Fever, itching, cough and constipation	☐
Muscle, bone or joint pain	☐
Decreased appetite, nausea	☐
Cough or shortness of breath	☐
Underactive thyroid (hypothyroidism)	☐

Help paying for Keytruda

Official manufacturer programs:

Merck Access Program: Benefits checks, prior-authorization help and copay help (copay help is for commercially insured patients).
Merck Patient Assistance Program: Free medicine for eligible uninsured or underinsured patients below income limits.

Copay cards are usually only for people with commercial (job or marketplace) insurance. If you have Medicare or Medicaid, look at free-drug programs and these independent foundations:

PAN Foundation: Copays and coinsurance, including for people with Medicare, when a disease fund is open.
HealthWell Foundation: Grants for copays, deductibles and premiums when a fund is open.
CancerCare Co-Payment Assistance Foundation: Help with treatment copays for insured people under income limits.
Good Days: Copay help and some travel costs.
NeedyMeds: Free database of assistance programs, open to anyone.

📋 Medicare: Medicare Part B usually covers drugs given by infusion in a doctor's office or hospital outpatient clinic. You typically pay 20% after the Part B deductible. See Medicare and your cancer drugs.

More: Our guide to paying for cancer treatment.

Regimens that include Keytruda

Carboplatin + pemetrexed + Keytruda

🏷 Tips from the maker's patient site

There is also a version given as a shot under the skin (Keytruda Qlex) in the stomach area or thigh, every 3 or 6 weeks. If you can become pregnant, expect a pregnancy test first, and use birth control during treatment and for 4 months after the last dose.

Summarized in our words from keytruda.com, which follows the FDA-approved patient information.

💬 What experts are saying

OncLive reports that on February 10, 2026, the FDA approved Keytruda with weekly paclitaxel (with or without Avastin) for some people with platinum-resistant ovarian cancer whose tumors test positive for PD-L1. A tumor test is used to see who may qualify.

Source: KEYNOTE-B96 Approval Reinforces Shift Toward Biomarker-Driven Treatment in Recurrent PROC (OncLive, March 5, 2026)

OncLive covers a June 2026 FDA approval of Trodelvy (sacituzumab govitecan) plus Keytruda as a first treatment for some people with advanced triple-negative breast cancer whose tumors are PD-L1-positive.

Source: FDA Approves Sacituzumab Govitecan Monotherapy and In Combination With Pembrolizumab in Frontline TNBC (OncLive, June 24, 2026)

▶ Watch

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❓ Questions to ask your doctor

What is the goal of this treatment for me?
How will we know if it's working?
Which side effects should I call about right away, and what number do I call after hours?
Which medicines, vitamins or supplements should I avoid?
Will this affect my ability to work, drive or care for my family?
What will this cost me, and who can help me with coverage?

Support groups

LUNGevity: Patient education, support groups and survivorship programs
GO2 for Lung Cancer: Helpline, biomarker testing support and navigation
Susan G. Komen: Breast care helpline, treatment assistance and education
Living Beyond Breast Cancer: Helpline, peer support and programs for metastatic breast cancer
Fight Colorectal Cancer: Patient guides, advocacy and community
Colorectal Cancer Alliance: Helpline (877-422-2030) and patient navigation
CancerCare: Free counseling, support groups and financial help
Cancer Support Community: Support groups, a helpline and the MyLifeLine online community

Common questions

Does Keytruda cause hair loss?

Hair loss is not one of the most common side effects listed for Keytruda alone. When Keytruda is given with chemo, hair loss is common because of the chemo. Ask your team what to expect from your exact regimen.

Does Keytruda cause weight gain?

Weight gain is not a listed common side effect. But Keytruda can cause thyroid problems, and an underactive thyroid can cause weight gain and tiredness. Tell your team about changes in your weight.

Does Keytruda cause neuropathy?

Nerve problems are not among the most common Keytruda side effects, but immune-related nerve problems are possible. If Keytruda is given with chemo like paclitaxel or carboplatin, the chemo is a more common cause of neuropathy.

Does Keytruda cause fatigue?

Yes. Tiredness is one of the most common side effects in the FDA label.

Does Keytruda cause joint pain or diarrhea?

Yes, both are listed. Report diarrhea right away, since it can be a sign of colitis (colon inflammation).

How much does Keytruda cost?

Cost depends on your insurance. Merck's access program can check your benefits and copay help, and the Merck Patient Assistance Program may give free medicine to eligible patients. See our paying-for-treatment guide.

📄 The official FDA label for Keytruda, including its patient information section, is on DailyMed.

Written by the Cancer Side Effects Guide team from FDA labels and leading cancer centers. Not medical advice. · Last updated: October 15, 2026 · How we write

Not medical advice. This page is general information to help you talk with your care team. It can't replace advice from your doctor or nurse. If you have a fever of 100.4°F (38°C) or higher during treatment, or any symptom that worries you, call your care team now. In an emergency, call 911.