Antigout agents (xanthine oxidase inhibitors)
Xanthine oxidase inhibitors block the enzyme that makes uric acid, which lowers uric acid levels in the blood and urine. Allopurinol and febuxostat prevent gout attacks and tophi, and allopurinol also prevents high uric acid during cancer treatment and certain kidney stones. Gout flares can increase when treatment starts, and rash, nausea, and diarrhea can occur. Allopurinol can cause rare but life-threatening skin reactions, more often in people who carry the HLA-B*5801 gene, so any rash is a reason to stop it and seek care.