Immunotherapy Side Effects: What's Common as of 2026?

Rash, diarrhea, and fatigue are common with immune checkpoint inhibitors, and in rare cases the immune system can attack normal organs, causing inflammation in the thyroid, lungs, intestines, liver, and elsewhere. Because these reactions can occur both during and after treatment, the general rule is to notify your medical team immediately about any new symptom, even a mild one.

Key Takeaways

• The most common side effects are rash, diarrhea, and fatigue (U.S. National Cancer Institute, NCI).
• Less commonly, patients may experience dermatitis, pneumonitis (interstitial lung disease), colitis, diabetes caused by pancreatitis, hepatitis, hypophysitis, myocarditis, nephritis, thyroid dysfunction, and neurological problems (NCI).
• Among immune-related inflammatory reactions, relatively common ones involve the colon, the endocrine system (pituitary, adrenal glands, thyroid, pancreas), the musculoskeletal system, the lungs (pneumonitis), and the skin — most of which can be controlled with steroids (NCI).
• Hormonal problems are usually permanent and may not reverse even after stopping treatment (NCCN patient guidelines).
• Side effects can appear not only during treatment but also after it ends, which is why NCCN recommends staying alert for new symptoms for at least one year after treatment finishes.

How Do Immunotherapy Side Effects Differ From Those of Conventional Chemotherapy?

• Immune checkpoint inhibitors don't attack cancer cells directly. Instead, they release the brakes on immune cells so the immune system attacks the cancer itself — which means side effects also tend to take the form of 'the immune system attacking normal organs.'
• This differs in character from the hair loss and bone marrow suppression seen with cytotoxic chemotherapy. Inflammation can occur in nearly any organ: the skin (color changes, rash, itching), lungs (cough, chest pain), colon (abdominal pain, diarrhea), pancreas (diabetes), liver, pituitary gland, heart, kidneys, and nervous system (muscle weakness, numbness, shortness of breath) (NCI).
• Just as the treatment's effects can appear late or continue even after treatment ends, a key feature of side effects is that they, too, can emerge well after treatment has finished.

What Symptoms Require Immediate Contact? — A Step-by-Step Check

1. Step 1: Digestive system — Contact your care team immediately if you have diarrhea several times a day, severe abdominal pain, or black or bloody stools.
2. Step 2: Respiratory system — Shortness of breath, worsening cough, or chest pain should prompt a check for possible interstitial lung disease.
3. Step 3: Endocrine system — Weakness, constipation, dry skin, weight gain, and increased sensitivity to cold can be signs of thyroid dysfunction.
4. Step 4: Systemic and neurological symptoms — A fever of 38°C or higher, sudden high blood pressure, confusion, hallucinations, seizures, skin blistering or rapidly spreading rash, and yellowing of the eyes or skin are emergency warning signs.
5. Step 5: If you can't reach your care team, go to the nearest emergency room and inform staff that you are being treated with an immune checkpoint inhibitor.

How Are Side Effects Monitored and Managed in Korea?

• Korean hospitals run blood tests before starting treatment to check thyroid, liver, and kidney function, and repeat these tests before each treatment cycle — delaying or adjusting dosing if abnormalities appear.
• Common side effects like rash and diarrhea are managed on an outpatient basis with topical treatments or antidiarrheal medication, while the medical team decides whether steroid treatment or hospital admission is needed if immune-related inflammation is suspected.
• International patients are advised to obtain an English-language summary listing the drug name and the date of the last dose before returning home, so local physicians can be informed of the immunotherapy history. It's also recommended to report any new symptoms to both the Korean care team and local physicians for about a year after treatment ends.

FAQ — My diarrhea won't go away. Could it be from immunotherapy?

Diarrhea and colitis are listed among the common side effects of immune checkpoint inhibitors (NCCN patient guidelines). If abdominal pain or black/bloody stools accompany the diarrhea, colitis is a possibility — don't wait it out, and contact your care team right away. Self-adjusting antidiarrheal medication on your own is not recommended.

FAQ — Can side effects appear even after treatment has ended?

Yes, they can. Side effects from immunotherapy can appear at any point during or after treatment. The NCCN patient guidelines recommend staying alert for new symptoms for at least one year after treatment ends, carrying an immunotherapy wallet card, and informing any other physicians of your treatment history. In particular, hormone-related problems involving the thyroid or adrenal glands can be permanent, making regular follow-up testing necessary.

Sources and Notes

• This article was written based on notes on immunotherapy and targeted therapy side effects (U.S. National Cancer Institute NCI, NCCN patient guidelines, National Cancer Information Center) along with immunotherapy reference materials.
• This content is intended for general medical information purposes only. Diagnosis, treatment approach, and outcomes may vary depending on individual circumstances. Please consult your medical team to determine the appropriate diagnosis and treatment for your specific condition.