Skin reactions are among the most common immune-related adverse events associated with pembrolizumab, affecting a significant proportion of patients during treatment. Most are mild and manageable, but a small number of skin reactions are serious and require prompt medical attention. Knowing what to look for makes a meaningful difference.
What skin reactions look like
The most typical pembrolizumab skin reaction is a rash - usually a generalised, maculopapular eruption (flat red patches or raised bumps, or both together) that can appear anywhere on the body. It is often mildly itchy. Vitiligo - patches of skin that lose pigmentation - is another immune skin reaction that occurs particularly in patients with melanoma. Vitiligo associated with pembrolizumab is generally considered a sign of immune activation and is often associated with response to treatment.
Less commonly, patients develop more severe reactions including blistering or involvement of mucous membranes. These require immediate medical attention.
Grading and treatment
Skin reactions are graded by severity. Grade 1 rash affects a small percentage of body surface area and causes minimal symptoms - it is typically managed with topical corticosteroid cream and antihistamines, and pembrolizumab continues. Grade 2 rash affects a larger area or is more symptomatic - it may prompt a temporary hold on treatment and a short course of oral steroids. Grade 3 reactions are severe and widespread, affecting more than 30% of body surface area or causing significant symptoms - they require systemic corticosteroids and a treatment hold. Grade 4 skin reactions - including Stevens-Johnson syndrome and toxic epidermal necrolysis - are life-threatening and require emergency hospital treatment and permanent discontinuation of pembrolizumab.
Injection site reactions
With subcutaneous pembrolizumab, some patients experience local reactions at the injection site - redness, swelling, or discomfort. These are typically mild, self-limiting, and distinct from systemic immune-related skin reactions. Rotating the injection site between visits helps reduce localised skin effects.
When to seek urgent help
Contact your clinical team promptly if you develop any blistering, if your rash spreads rapidly, if you have mouth ulcers or any involvement of your lips or eyes, or if your skin reaction is accompanied by fever or other systemic symptoms. These features can indicate a serious reaction that needs same-day assessment.
Questions about managing side effects at home?
Side effect monitoring is built into every visit. Start with the eligibility check to find out whether home treatment is right for you.
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