The authors observed an interaction between allopurinol and both theophylline and warfarin in 2 patients. To determine the possible effect of allopurinol on hepatic oxidative metabolism a [14C]-aminophenazone (aminopyrine) breath test was performed before and during treatment with allopurinol 100mg daily in 5 patients with hyperuricaemia. Allopurinol prolonged the [14CO2]-aminophenazone half-life from 72 ± 13 to 104 ± 16 minutes (mean ± SEM, p < 0.05). It is possible that allopurinol may produce clinically significant drug interactions through an inhibitory effect not only on xanthine oxidase metabolism but also on oxidative metabolism.