The short answer
NCI says immunotherapy side effects can occur at any point during and after treatment, and that nobody can predict them. Its organ-inflammation page asks you to keep the drug name and your oncologist's and hospital's contact details written down, in case you need an emergency department.
NCI says side effects can occur at any point during and after treatment, and that doctors and nurses cannot know for certain when or if they will occur or how serious they will be.
NCI lists colitis, pneumonitis, hepatitis, nephritis, myocarditis, uveitis, pancreatitis, myositis, and inflammation of the pituitary, adrenal and thyroid glands among the organ effects.
NCI says brain inflammation, encephalitis, may look like mild flu at first or may bring sudden high fever, confusion, hallucinations, seizures and vomiting.
NCI says your oncologist may want to hear about inflammatory symptoms first, or may tell you to go straight to emergency care - ask which applies to you.
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The full explanation.
The short answer
Immunotherapy helps your immune system act against cancer. It can also trigger inflammation in normal, healthy tissue. You do not need to memorize every possible side effect. What matters most is having a clear plan for which symptoms your team wants to hear about right away, and which need emergency care.
Why these side effects are different
Many cancer treatments mainly damage fast-growing cells. That is why hair loss and mouth sores are common with chemotherapy. Immunotherapy side effects work differently. Your activated immune system can start affecting a healthy organ, not just the cancer. That organ can be your skin, lungs, colon, liver, hormone glands, kidneys, nerves, or another part of the body. Doctors call these immune-related adverse events.
Timing can be unpredictable
Immunotherapy side effects can start at almost any point. The National Cancer Institute notes they can occur during treatment or after it has ended. A new symptom weeks or months after your last dose can still be related to immunotherapy. Read new symptoms with your treatment history in mind, not just how you feel today.
Symptoms worth watching by organ system
NCI sets these out organ by organ.
- Digestive system. Colitis brings stomach or belly pain, diarrhea, and black or bloody stools. Some people have mild hepatitis, picked up only on a blood test.
- Hormone glands. Pituitary inflammation is the commonest of these and causes headaches and fatigue. Adrenal inflammation causes fatigue, muscle weakness, appetite loss, weight loss and belly pain. An underactive thyroid brings weakness, constipation, dry skin, weight gain and feeling cold; an overactive one brings diarrhea, weight loss, sweating and heat sensitivity.
- Pancreas. Severe belly pain with nausea and vomiting.
- Lungs. Pneumonitis makes you short of breath and gives you a bad cough.
- Skin. Rashes, itching, blisters and sores.
- Liver. Hepatitis yellows the skin and eyes, and can bring dark urine, nausea, stomach pain and easy bleeding.
- Kidneys. Nephritis can cut how much urine you make, or put blood in it.
- Heart, eyes and nerves. Myocarditis can drop your blood pressure. Uveitis or episcleritis can change vision or cause eye pain. Nerve inflammation causes tingling, numbness or weakness in the hands, feet and sometimes the face.
NCI notes that your doctor will check hormone levels regularly, because these gland problems often show on a blood test before you feel them.
Which symptoms cannot wait
Some symptoms need same-day contact with your team, not a wait for your next visit. These include new or worsening diarrhea, black or bloody stools, a new cough or breathlessness, a widespread or blistering rash, yellowing skin or eyes, eye pain or blurred vision, and passing much less urine than usual.
A few symptoms need emergency care straight away rather than a phone call first. Severe shortness of breath, chest pain, fainting, and signs of a severe allergic reaction such as swelling of the face or throat all belong in that group. So does the picture NCI describes for encephalitis, inflammation of the brain: a sudden high fever with confusion, hallucinations, seizures or vomiting.
NCI is explicit that this split is personal. Your oncologist may want to be called first about inflammatory symptoms, or may tell you to go straight to emergency care. Ask which, and write the answer down.
What to put in your call plan
NCI names three things to write down and keep handy: the drug name or names, your doctor's contact information, and your hospital's contact information. That note is for the emergency room, where nobody knows your treatment history.
Ask your team for a written list to go with it. It should cover which symptoms need a same-day call, the number to use after hours, and what to tell urgent-care clinicians about your immunotherapy. Immune-related inflammation is managed differently from an ordinary infection, often with steroids that damp the immune response down, so the clinician treating you needs to know what you are on.
What not to assume
Do not assume a new symptom is harmless just because it seems unrelated to your cancer. Do not assume every new symptom is immune-related either. Infections and medicine side effects still happen during immunotherapy. Sorting this out is your care team's job. Reporting the symptom promptly is yours.
How this fits with your overall treatment
How a side effect is managed depends on which organ is involved and how severe it is. Your team weighs the benefit of continuing immunotherapy against the risk of the side effect. This is part of why prompt reporting matters: catching a problem early, while it is mild, generally gives your team more ways to manage it well.
What to ask your care team
- Which symptoms should I report the same day?
- Which symptoms should lead to urgent or emergency care if I cannot reach you?
- Do I need to carry an immunotherapy wallet card or medication list?
- If I develop a side effect, what does treatment for it usually involve, and would it change my immunotherapy schedule?
Sources
Words to know
Tap any term to see what it means.
Common questions
Can immunotherapy side effects happen after treatment stops?
Yes. NCI says side effects can occur at any point during and after treatment, and that doctors and nurses cannot know for certain when or if they will occur or how serious they will be.
Are immunotherapy side effects always mild?
No. NCI says these side effects may be serious and even life-threatening, and that some types of immunotherapy can cause severe or fatal allergic and inflammation-related reactions, though those reactions are rare.
What does NCI say to have written down?
The drug names, your doctor's contact information, and your hospital's contact information, kept handy so you can show it to clinicians in an emergency room who do not know you are on immunotherapy.
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).
Your next step
Turn symptoms, timing, and questions into a short visit plan.
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Sources last checked: 2026-08-13 what this meansLast updated: 2026-08-13Next planned review: 2027-02-03
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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: 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.
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