The influence of allopurinol and size of dose on the metabolism of phenylbutazone in patients with gout.

Administration of allopurinol 300 mg/day produced minimal changes in the disappearance of phenylbutazone in each of five subjects following single doses (6 mg/kg) in clinical range and caused some prolongation (21%, 52%) of drug half-lives in two of six subjects after single small doses (0.5 mg/kg); mean half-life was not significantly altered by allopurinol at either dose level (means of 52 versus 48 at 0.5 mg/kg and 68 versus 70 h at 6 mg/kg). Size of dose altered half-life when phenylbutazone was used alone; three subjects showed considerably longer half-lives at the higher dose (86 versus 47, 91 versus 41, 65 versus 38 h). Allopurinol caused a greater than 50% prolongation of half-lives in two of five subjects who received single 0.5 g doses of probenecid. These preliminary data do not indicate a need to change the dose of phenylbutazone when subjects are receiving allopurinol.