Excess of Ampicillin Rashes Associated with Allopurinol or Hyperuricemia

Abstract

In a comprehensive drug surveillance system, drug rashes were observed among 22.4 per cent of 67 hospitalized medical patients receiving allopurinol and ampicillin concomitantly, and among 7.5 per cent of 1257 patients receiving only ampicillin. The relative risk for allopurinol recipients, as compared with nonrecipients, was 3.0, with 95 per cent confidence limits of 1.8 and 4.6. Potentiation of ampicillin rashes by allopurinol (or hyperuricemia) seems a likely explanation, since only 2.1 per cent of 283 patients receiving allopurinol without ampicillin experienced drug rashes – a rate comparable with that observed among ampicillin nonrecipients in general. Data on uric acid levels were not available, and it is not clear whether the potentiation should be ascribed to allopurinol or to hyperuricemia.