The short answer
Checkpoint inhibitors can inflame the lungs. Pneumonitis shows up as a new or worsening cough, breathlessness at less effort than before, or chest pain, and it is inflammation rather than an infection, though the two can look alike. Report it against your own recent baseline, and name the drug at any urgent visit.
Immune-mediated pneumonitis occurred in 3.4% of patients in pembrolizumab labeling, and was fatal in 0.1%.
Measure against your own recent baseline: stopping halfway up stairs you used to climb easily is the change to report.
Pneumonitis is inflammation, not an infection, so steroids rather than antibiotics settle it — but a chest infection can look identical or sit alongside it, so name your checkpoint inhibitor at any urgent visit and let the team sort out which it is.
Some cases are found on scans before symptoms, so ask whether pneumonitis is being considered if lung changes are mentioned.
Choose how you want to understand this
The full explanation.
Immune checkpoint inhibitors help your immune system attack cancer. They work by removing some of the controls that normally hold it back. Sometimes the immune system also turns on healthy organs. The National Cancer Institute (NCI) calls this organ-related inflammation. It names each type after the organ, using the ending "-itis." Inflammation in the lungs is called pneumonitis.
What pneumonitis feels like
NCI describes it in one sentence: "Inflammation in the lungs, called pneumonitis, can cause you to feel short of breath and have a bad cough."
That is a short list, and the words matter. Two things count here:
A new cough, or a cough that is getting worse. The patient information from the U.S. National Library of Medicine for pembrolizumab lists "New or worsening cough, shortness of breath, chest pain" among the symptoms that need urgent attention. A cough you have had for years is not the signal. A cough that started after you began immunotherapy is. So is one that has changed.
Breathlessness at less effort than before. Use this measure, because "short of breath" is easy to talk yourself out of. Compare against your own recent baseline. Say last month you climbed a flight of stairs without stopping, and this week you stop halfway. That is the change worth reporting, even if you can still get up the stairs. Getting breathless walking across a room, or while sitting still, is further along the same line.
Chest pain or chest discomfort appears alongside cough and breathlessness in the same list of symptoms to act on.
Why it can be quiet at the start
Pneumonitis does not always announce itself. In FDA labeling for pembrolizumab, immune-mediated pneumonitis occurred in 3.4% of patients treated, including fatal cases in 0.1%. Some cases are picked up on imaging done for other reasons, before the person has described many symptoms.
There are two consequences for you. First, does a scan report mention changes in your lungs? Ask your oncology team directly whether pneumonitis is being considered, even if you feel well. Second, do not use "I feel mostly fine" as a reason to hold back a mild new cough. Early pneumonitis and mild pneumonitis can look the same from the inside.
How fast to report it
The instruction in the National Library of Medicine's pembrolizumab information is not gentle, and it should not be softened here: "Some side effects can be serious. If you experience any of these symptoms, call your doctor immediately or get emergency medical treatment." New or worsening cough, shortness of breath, and chest pain are on that list.
The same phrasing appears in the ipilimumab patient information. There, "cough, chest pain, or shortness of breath" is followed by the instruction to "call your doctor immediately or get emergency medical treatment."
NCI adds that your oncologist "may advise you to call him or her first about inflammatory symptoms, or you may be advised to seek emergency medical care." Find out at your next visit which of these your team wants. Ask where to call outside clinic hours too. Then you are not deciding it while breathless.
If you go to an emergency department or urgent care, say the name of your immunotherapy drug and when you last received it. Clinicians who do not know you are on a checkpoint inhibitor may settle on pneumonia or a chest infection and look no further. Pneumonitis is inflammation rather than infection, so antibiotics alone will not clear it. Infection, a clot, fluid and the cancer itself can cause the same picture, or sit alongside pneumonitis, so the team may treat for more than one cause while the tests come back.
Why steroids
The underlying problem is an immune response that has gone too far. So the treatment targets the immune system. NCI states: "Sometimes these side effects are managed with medicines, such as steroids, that work to slow down an overactive immune response."
In pembrolizumab labeling, systemic corticosteroids were required in 67% of patients who developed pneumonitis. The labelled approach is a steroid such as prednisone, at an amount your team works out from your weight and how severe the inflammation is. It carries on until the reaction has largely settled, then comes down slowly over at least a month. Both the size of the dose and the speed of the taper are the team's decisions, and the taper is not one to shorten on your own. Pneumonitis led to the drug being permanently discontinued in 1.3% of patients, and temporarily withheld in 0.9%.
After your last dose
Lung symptoms can begin after treatment has ended. FDA labeling states that "immune-mediated adverse reactions can occur at any time after starting treatment with a PD-1/PD-L1 blocking antibody." They usually appear during treatment, but they "can also manifest after discontinuation." A new cough or new breathlessness months after your final infusion still belongs to your oncology team.
Sources
- National Cancer Institute — Immunotherapy and Organ-Related Inflammation: https://www.cancer.gov/about-cancer/treatment/side-effects/organ-inflammation
- MedlinePlus (National Library of Medicine) — Pembrolizumab Injection: https://medlineplus.gov/druginfo/meds/a614048.html
- MedlinePlus (National Library of Medicine) — Ipilimumab Injection: https://medlineplus.gov/druginfo/meds/a611023.html
- U.S. Food and Drug Administration — KEYTRUDA (pembrolizumab) prescribing information: https://www.accessdata.fda.gov/drugsatfda_docs/label/2024/125514s158lbl.pdf
Words to know
Tap any term to see what it means.
Common questions
What is pneumonitis?
Immune checkpoint inhibitors work by removing some of the controls that normally hold the immune system back. Sometimes the immune system also turns on healthy organs, which NCI calls organ-related inflammation and names after the organ using the ending -itis. Inflammation in the lungs is pneumonitis, and NCI says it can make you feel short of breath and have a bad cough.
Which cough should I report?
One that started after you began immunotherapy, or one that has changed or is getting worse. A cough you have had for years is not the signal. The pembrolizumab patient information lists new or worsening cough, shortness of breath and chest pain among the symptoms that need urgent attention.
How do I judge whether I am more breathless?
Compare against your own recent baseline, because short of breath is easy to talk yourself out of. If last month you climbed a flight of stairs without stopping and this week you stop halfway, that is the change worth reporting, even if you can still get up the stairs. Getting breathless walking across a room, or while sitting still, is further along the same line.
Do I call the team or go to the emergency department?
Ask in advance. NCI says your oncologist may advise you to call first about inflammatory symptoms, or may advise you to seek emergency medical care, so find out which your team wants and where to call outside clinic hours. If you do go in, say the name of your immunotherapy drug and when you last received it. Clinicians who do not know you are on a checkpoint inhibitor may settle on pneumonia and look no further. Pneumonitis is inflammation rather than infection, so antibiotics alone will not clear it, though a chest infection can look the same or run alongside it, and the team may cover for both while tests come back.
Can this start after my last dose?
Yes. FDA labeling states that immune-mediated adverse reactions can occur at any time after starting treatment with a PD-1/PD-L1 blocking antibody. They usually appear during treatment, but they can also manifest after discontinuation. A new cough or new breathlessness months after your final infusion still belongs to your oncology team.
Questions to ask your doctor
Being prepared helps you get the most out of your appointments. Save or print these questions.
Tap a question to save it to your list (kept on this device).
Speak With Trained Specialists & Human Navigators
Cancer Explained provides educational guidance, but does not replace trained specialists, social workers, or your medical team.
Talk to a trained cancer information specialist
Free, confidential assistance from NCI Cancer Information Service via phone, chat, or email.
Contact your oncology team
Locate after-hours contact numbers, portal messages, or urgent triage phone lines.
Find a patient navigator
Get one-on-one help with appointments, logistics, translation, and care coordination.
Find a genetic counselor
Discuss inherited mutation risk, family history, and genetic testing options.
Find an oncology social worker
Access emotional counseling, family support groups, and mental health resources.
Find a financial navigator
Locate copay assistance foundations, grant programs, and lodging/travel support.
Find a clinical-trial specialist
Search matching studies and speak with NCI trial information specialists.
Get urgent help
Immediate emergency guidance for fever (>100.4°F during chemo), severe pain, or shortness of breath.
Help Us Improve This Guide
Did this explanation answer your question and help you determine your next step?
Know someone who needs this?
Plenty of people are looking for something like this and do not know where to start. If this would help a friend or someone you love, send it on — we have written an opening line so you do not have to stare at an empty message. You can change every word of it.
Your message is written and sent in your own email or messaging app — we never see who you send it to, and nothing is added to any list.
Plain-language explanation of the published sources cited on this page. AI-assisted, source-checked, not clinician-reviewed.
Written by: Cancer ExplainedSources last checked: 2026-07-23 what this meansLast updated: 2026-08-19Next planned review: 2027-01-28
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 — Source checked. This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
General education — varies by person. Answers genuinely differ between people. This page explains what commonly varies and points you to your care team for your situation.
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.
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: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
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.
Read more about our editorial process, our use of AI, and our corrections policy.
Spotted a problem? Report an error — a factual mistake, broken or outdated source, confusing wording, or anything that seems unsafe. Please do not include names, medical record numbers, dates of birth, addresses, or other identifying medical information in your report.
After using this page, do you understand what to do next?
Anonymous — we only record the answer, never who gave it.
Related articles
Still have questions?
Educational answers, plain language
Free to print and share