Takes the brakes off your immune system so it attacks the cancer. Sometimes it attacks normal organs too.
The medicines
Tap the side effect you have on the map to see how severe it is and what to do. Not everyone gets every side effect, and most are mild.
From: Pembrolizumab
Tired, better with rest
Manage at home
Mild (grade 1)
Tired, but better with rest.
Moderate (grade 2)
Not better with rest; you cannot do housework, shopping or work as usual.
Severe (grade 3 or 4)
Cannot get out of bed or walk, fainting, dizziness on standing, or tiredness with breathlessness or confusion.
From: Skin rash and itching: Pembrolizumab; Bowel inflammation (colitis): Pembrolizumab; Lung inflammation (pneumonitis): Pembrolizumab; Liver inflammation (hepatitis): Pembrolizumab; Hormone glands (pituitary, adrenal): Pembrolizumab; Other organs (rare): Pembrolizumab
Mild (grade 1)
Mild rash or itching, 1 to 3 more stools than usual, or mild tiredness.
Moderate (grade 2)
4 to 6 more stools than usual, tummy pain, new cough or breathlessness on walking, rash over a large area, or headache.
Severe (grade 3 or 4)
7 or more stools a day or blood in stools, breathless at rest, yellow eyes, chest pain, severe headache or change in vision, confusion, extreme thirst, or severe weakness.
Uncommon but important. We look for these with blood tests, an ECG or scans.
From: Pembrolizumab
The immune system can attack the thyroid, making it overactive at first and then underactive.
When: Usually 1 to 3 months after starting.
What helps:
Check your symptoms in My treatment
General information. Doses are not shown. Your own doctor may advise differently. When to seek help urgently
Written and medically reviewed by Dr Prathyusha Eaga, MD, DM (Medical Oncology). Last reviewed: 12 October 2026. Based on NCCN and ESMO guidelines.
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