This page is for informational purposes only and does not replace medical advice. Please consult Dr Aditya Sarin or a qualified oncologist about your specific treatment plan and what side effects to expect. As of 2026, this reflects current clinical guidance.

If you’re about to start immunotherapy, you’ve probably already heard it described as “gentler” than chemotherapy. In many ways, that’s fair; many patients tolerate it well. But immunotherapy side effects work through a completely different mechanism than chemo side effects, and that difference matters. They can show up later than you’d expect, in places you wouldn’t necessarily connect to a cancer drug, and they need to be reported quickly rather than waited out. Here’s what’s actually going on in your body, what’s common, and what needs urgent attention.

Why immunotherapy side effects are different from chemotherapy

Chemotherapy damages fast-growing cells throughout the body, which is why it causes things like hair loss and nausea. Immunotherapy works through an entirely different mechanism. According to the National Cancer Institute, immunotherapy helps your immune system detect and destroy cancer cells more effectively, and many side effects happen because that revved-up immune system also attacks healthy cells and tissue. In other words, the same process that helps immunotherapy fight cancer also causes side effects, which is part of why oncologists can’t simply turn the treatment “down” the way they might adjust a chemo dose.

This immune overactivation is why immunotherapy side effects are officially called immune-related adverse events (irAEs) rather than just “side effects.” They resemble autoimmune conditions because, functionally, that’s close to what’s happening: your immune system, no longer held in check, starts treating parts of your own body as a threat.

Common early side effects

According to the American Cancer Society, the most frequently reported immunotherapy side effects include:

• Fatigue
• Skin changes, including rash or itching
• Flu-like symptoms such as fever, chills, and muscle aches
• Stomach issues like nausea, vomiting, or diarrhoea
• Soreness, redness, or swelling at the injection or infusion site

Most of these are mild and show up early in treatment. They’re also, importantly, not a sign the treatment isn’t working, and they’re not automatically a sign it is either. Report them to your care team regardless, since the same symptom can range from a minor nuisance to the first sign of something that needs medication.

Immune-related adverse events: What to know about the more serious ones

Beyond the common early symptoms, immunotherapy can affect specific organs in ways that need closer monitoring. Per guidance summarised by the National Comprehensive Cancer Network and echoed across major cancer centres, the organs most commonly affected include:

• Lungs (pneumonitis): New or worsening cough, shortness of breath, chest pain
• Gut (colitis): Diarrhoea, blood in stool, severe abdominal pain
• Liver (hepatitis): Yellowing of the skin or eyes, dark urine, right-sided abdominal pain, unusual fatigue
• Endocrine glands (thyroiditis, adrenal insufficiency): Unexplained weight change, feeling unusually cold or hot, extreme tiredness, dizziness
• Skin: Rash, blistering, painful sores

Thyroid problems are worth knowing about specifically. Immunotherapy-related thyroid damage is often permanent once it happens, meaning patients may need thyroid hormone replacement medication for life, according to oncology nurse practitioners who monitor these effects closely. It’s manageable, but it doesn’t reverse on its own once treatment ends.

Most immune-related adverse events, according to the Cancer Council, tend to appear several weeks or months after starting treatment rather than immediately, and in rare cases can appear even after treatment has finished. That delay is part of why ongoing monitoring matters throughout, not just at the start.

Immunotherapy and fatigue

Fatigue shows up with immunotherapy for different reasons than it does with chemo. It’s not usually tied to anaemia or bone marrow suppression the way chemo fatigue often is. Instead, it can be a general immune-system response, or, in some cases, an early sign of an endocrine problem like thyroid or adrenal involvement, both of which commonly cause tiredness.

That distinction matters practically. If fatigue during immunotherapy is new, worsening, or paired with other changes, weight shifts, feeling unusually cold, or dizziness, it’s worth flagging to your oncologist rather than assuming it’s simply treatment-related tiredness. A quick blood test can usually tell the difference.

When immunotherapy side effects need urgent attention

Immune-related adverse events can escalate faster than people expect, which is why prompt reporting matters more here than with most cancer treatments. Symptoms that need a same-day call to your oncology team, or emergency care if you can’t reach them, include:

• Severe or worsening shortness of breath, or new chest pain.
• Severe diarrhoea, or blood in your stool.
• Yellowing of the skin or eyes.
• Confusion, severe headache, neck stiffness, or vision changes.
• Severe muscle weakness or new numbness.
• A fever that doesn’t settle, especially alongside any of the above.

Even mild versions of these symptoms are worth mentioning early. Immune-related adverse events are far easier to manage when caught before they progress, and most are reversible with prompt treatment, typically corticosteroids to calm the immune response, when addressed quickly.

How your medical oncologist manages immunotherapy side effects

Because irAEs can appear late and affect almost any organ, monitoring during immunotherapy is often more structured than during chemo. A medical oncologist like Dr Aditya Sarin typically orders regular bloodwork to catch early thyroid, liver, or kidney changes before symptoms appear, asks about new or unusual symptoms at every visit, and has a clear plan for grading severity and stepping in with steroids or other treatment when something crosses the line from mild to concerning. This kind of active surveillance is part of what makes immunotherapy safer in practice than it might sound on paper.

The takeaway

Immunotherapy side effects work differently from chemo side effects, and understanding that difference, especially the delayed timing and the wide range of organs that can be affected, helps you know what’s worth reporting and when. If you’re considering immunotherapy or already partway through treatment, book a consultation with Dr Aditya Sarin to talk through what to expect and how your specific treatment plan will be monitored.

References

• National Cancer Institute: Immunotherapy to Treat Cancer
• National Cancer Institute: New Drugs, New Side Effects- Complications of Cancer Immunotherapy
• American Cancer Society: Immunotherapy for Cancer
• Cancer Council Australia: Side Effects of Checkpoint Inhibitors
• Roswell Park: Side Effects of Checkpoint Inhibitor Immunotherapy