Effect of diflunisal on the human plasma levels and on the urinary excretion of naproxen.

A double-blind placebo controlled crossover study was performed in twelve healthy volunteers receiving multiple doses of naproxen (250 mg b.i.d.) and diflunisal (250 mg b.i.d.) to study the effect of diflunisal co-administration on the naproxen levels in plasma and urine. The naproxen steady-state level was reached between the second and the third day. The peak plasma level appeared about 2 hr after drug administration and varied from 68 to 75 microgram/ml. This maximum was not modified by a simultaneous administration of diflunisal neither in its position nor in its intensity. Statistical analysis using a parametric method, did not reveal a significant influence of diflunisal on the plasma kinetics or on the urinary elimination of naproxen.