Pembrolizumab Side Effects: What to Watch For

The most commonly reported side effects of pembrolizumab are tiredness, itching, a skin rash, loose motions, nausea or reduced appetite, joint and muscle aches, cough, fever and constipation. Tiredness is the one people report most often, affecting roughly one in three patients on single-agent treatment. Most of these are mild to moderate and are managed without stopping treatment.

• Tiredness — the most frequent effect, and often the hardest to describe. It is worth flagging if it is new or suddenly worse, because persistent tiredness is also how thyroid and adrenal problems announce themselves. See fatigue during immunotherapy.
• Itching and skin rash — usually the earliest reaction of all. Mild rash is common; a rash with blisters, peeling, or sores in the mouth is not, and is treated as urgent.
• Loose motions — a small increase over your normal bowel habit is common. Counting is what separates this from colitis; see counting loose motions accurately.
• Nausea, reduced appetite, constipation — usually mild, and usually managed with supportive medication your team prescribes.
• Joint and muscle aches — common, and occasionally the start of an inflammatory arthritis or myositis that needs treatment in its own right.
• Cough — often unremarkable, but a new or changing cough is also the first sign of lung inflammation, so it is always reported rather than watched.

A mild symptom is not automatically a minor one. Almost every serious immune reaction on this page starts as something small. That is the reason the advice is always to report a new symptom to the oncology team rather than judge its severity at home.

The serious side effects of pembrolizumab are the immune-related adverse events — inflammation of the bowel, lungs, liver, heart, pituitary, adrenal glands, kidneys, pancreas or skin. Severe reactions occur in a minority of patients on single-agent PD-1 blockade, according to management guidance from ASCO and ESMO. They can progress quickly. They are treated with corticosteroids, not at home.

• Colitis — inflammation of the bowel. Frequent loose motions, cramping, or blood or mucus in the stool. Do not self-treat with anti-diarrhoeal medicine; contact the oncology team the same day.
• Pneumonitis — inflammation of the lungs. New or worsening cough, or breathlessness on activity that was easy last week. This is not managed at home; it needs same-day assessment.
• Hepatitis — inflammation of the liver. Usually silent and picked up on routine blood tests before a cycle, which is why those tests are not optional.
• Myocarditis — inflammation of the heart muscle. Rare, but the most dangerous reaction on this list. Chest pain, palpitations or breathlessness at rest means going to the nearest emergency department immediately.
• Hypophysitis and adrenal insufficiency — the pituitary or adrenal glands stop producing hormones. Severe headache, profound weakness, dizziness on standing, vomiting. An adrenal crisis is a medical emergency and is treated in hospital, not at home.
• Thyroid inflammation — the most common endocrine reaction. Often begins with a brief overactive phase before settling into an underactive thyroid that needs lifelong tablets.
• Nephritis — inflammation of the kidneys, usually detected as a rising creatinine on blood tests rather than as a symptom.
• Immunotherapy-induced diabetes — uncommon, sudden, and permanent. Excessive thirst, passing large volumes of urine and rapid weight loss need urgent testing.
• Severe skin reactions — blistering, peeling skin, or ulcers in the mouth or eyes. Treated as an emergency.

Different organs are affected at different points in treatment, and that pattern is one of the most useful things to know. Skin reactions come first, usually within two to six weeks. Thyroid, bowel and liver reactions follow over the next two to three months. Pituitary, adrenal and kidney problems tend to arrive later. Myocarditis is the exception — rare, but usually early.

Immune-related reactions are found through routine blood tests before every cycle and through what the patient reports between cycles. Liver enzymes, kidney function, thyroid hormones, blood counts and often blood sugar are checked each time. Two of the most serious reactions — hepatitis and nephritis — are usually silent, and are caught only this way.

When a reaction is confirmed, treatment follows a well-established pattern set out by ASCO, ESMO and NCCN. Mild reactions may be managed with supportive care while immunotherapy continues. Moderate ones usually mean holding the next dose and starting corticosteroids. Severe ones mean stopping the dose, admitting the patient, and using higher-dose steroids, with a second immunosuppressant added if the reaction does not settle. Steroids are then tapered slowly over weeks, not stopped abruptly.

Whether treatment can resume afterwards depends on which organ was affected and how severe the reaction was. Some reactions permit a careful rechallenge; myocarditis and severe pneumonitis usually do not. Related reading: steroids for immunotherapy side effects, why steroids are tapered slowly, and rechallenge after an immune reaction.

At CION, immunotherapy is administered as day care, and response-assessment PET-CT is coordinated at partner imaging centres rather than performed in-house.

No. Side effects are not a progress report. Some studies have observed an association between certain immune-related reactions and response, but the link is not reliable for any individual patient. People respond with almost no side effects. People develop difficult reactions and see no benefit.

Whether treatment is doing what it is intended to do is judged on response-assessment imaging and clinical review, on a schedule your oncologist sets — not on how unwell you feel in week six. Having no side effects is not a reason to worry, and having them is not reassurance. See do side effects mean immunotherapy is working and no side effects from immunotherapy.

Yes. Pembrolizumab changes how the immune system behaves, and that change outlasts the final infusion. Immune-related reactions have been reported weeks and, less often, months after a last dose. Thyroid and pituitary damage can be permanent, leaving a person on hormone replacement long term.

The practical consequence is simple: anyone who has received a checkpoint inhibitor should tell every doctor who treats them afterwards, including in an emergency department, that they had immunotherapy and roughly when. A doctor who does not know that will not think of an immune reaction. See delayed immune reactions after stopping, monitoring after stopping immunotherapy, and always tell doctors about immunotherapy.

Long-term data on immune-related effects years after treatment is still maturing. Where the honest answer is that we do not yet know, that is what a treating team should say.

Much of what is on this page applies to the PD-1 and PD-L1 class rather than to this molecule alone. If a nivolumab-based regimen is the one in front of you, Nivolumab (Opdivo / Opdyta): What It Is and How It Is Used covers the same ground for that drug, and Nivolumab Cost in India After the Biosimilar Launch explains why its price picture changed in 2026 while pembrolizumab's has not.

For which molecules hold Indian regulatory approval, and for which indications, see Which Checkpoint Inhibitors Are Approved in India?. For the wider set of medicine pages, return to Immunotherapy Medicines Explained, or start from the immunotherapy pillar — including immune-related adverse events explained and the side-effects overview.