Common Allergic Reactions to Keytruda
Keytruda (pembrolizumab), an immunotherapy drug for cancers like melanoma and lung cancer, can trigger allergic responses, often during or shortly after infusion. Symptoms typically mimic infusion reactions and include rash, itching, flushing, chills, fever, and shortness of breath. These occur in about 13% of patients, mostly mild to moderate.[1][2]
Severe Symptoms Requiring Immediate Attention
Watch for signs of serious hypersensitivity, such as swelling of the face, lips, or throat (anaphylaxis), rapid heartbeat, low blood pressure, wheezing, severe dizziness, or trouble swallowing. These demand stopping the infusion and seeking emergency care, as they affect up to 1-2% of patients.[1][3]
How Symptoms Differ from Other Keytruda Side Effects
Allergic reactions happen fast—within hours of dosing—unlike immune-related side effects like colitis or pneumonitis, which develop over days or weeks and involve organ inflammation (e.g., diarrhea, cough). Infusion reactions may overlap but are distinguished by their acute timing.[2][4]
Risk Factors and Onset Timing
Patients with prior reactions to monoclonal antibodies face higher risk. Symptoms usually start during the first or second infusion, peaking within 30 minutes to 4 hours. Pre-medication with antihistamines or steroids reduces incidence in at-risk cases.[1][3]
What to Do if Symptoms Appear
Stop treatment and notify your doctor immediately. Management includes pausing infusion, giving epinephrine for anaphylaxis, or supportive care like oxygen. Most reactions resolve without long-term issues, but rechallenge is rare.[2][4]
[1]: Keytruda Prescribing Information (Merck)
[2]: FDA Label for Pembrolizumab
[3]: NCCN Guidelines: Infusion Reactions
[4]: Cancer.gov: Immunotherapy Side Effects