A drug allergy should always be suspected in a patient presenting with a whole-body rash and eosinophilia, especially if a new medication was recently started. Allopurinol-induced DRESS syndrome is associated with significant mortality (10%).

If you develop a rash (even a mild one) while taking allopurinol, stop taking it and contact your prescriber immediately. If you experience severe allopurinol side effects, like swelling in the face or lips or difficulty breathing, call 911 or seek emergency care.

Skin rashes. Stevens-Johnson syndrome is a rare side effect of allopurinol. It causes flu-like symptoms, followed by a red or purple rash that spreads and forms blisters. The affected skin eventually dies and peels off. It's more likely to happen in the first 8 weeks of taking allopurinol, or when the dose is increased too quickly.

During the treatment with allopurinol, patients may commonly experience rash, gout flares, gastrointestinal symptoms, elevation in hepatic enzymes, hematologic abnormalities and drowsiness .