The effect of acetylsalicylic acid (1500 mg/day), phenylbutazone (300 mg/day), paracetamol (1500 mg/day) and tolfenamic acid (300 mg/day), administered for 3 days, on serum concentrations of phenytoin and carbamazepine were studied in patients on continuous antiepileptic therapy. When measured 10 h after the last dose of the analgesics, the only significant effect was a decrease in total serum phenytoin after 3 days of phenylbutazone. When 6 patients on continuous phenytoin therapy took phenylbutazone for 2 wk there was at 2 days an initial decrease followed by a significant increase in total serum phenytoin and a concomitant increase in free phenytoin in serum. Phenylbutazone was well tolerated by most patients; it was discontinued in 1 patient due to obvious signs of phenytoin intoxication with concomitant increases in the serum free and total phenytoin concentrations.
