Influence of dextropropoxyphene on steady state serum levels and protein binding of three anti-epileptic drugs in man.

Interactions between analgesics and anti-epileptic drugs may sometimes present a serious clinical problem. The aim of the study was to investigate the influence of usually applied doses of dextropropoxyphene (DPX) on the steady state levels of carbamazepine (CBZ), phenytoin (DPH) and phenobarbital (PB). Sixteen patients in monotherapy completed the trial, while four patients dropped out. In patients on CBZ serum levels increased (mean appr. 66%) after 6 days on DPX. In three of the patients a further increase was seen after an additional week on DPX. One patient discontinued the DPX intake because of clinical signs of toxicity, but the remainder were clinically unaffected. CBZ-epoxide levels declined simultaneously. For DPH only a doubtful increase was observed after 1-2 weeks on DPX. For PB an average increase of 20% in serum level was noted after 1 week. The protein binding of CBZ and DPH was not affected. It is concluded that patients on CBZ should be treated only with DPX if monitored properly. Patients on DPH or PB should be followed carefully until further evidence has been produced.