Actions and interactions of indomethacin and diazepam on performance in healthy volunteers

Objective and subjective effects on performance of single oral doses of indomethacin (IM) 50 mg and 100 mg and diazepam (DZ) 10-15 mg, alone and in combination, were investigated in two double-blind studies conducted with parallel groups of healthy drug-naive student volunteers. Objective and subjective effects were measured at baseline as well as 0.5 and 1.5 hours after treatment. DZ significantly impaired performance in digit symbol substitution, letter cancellation, tracking and flicker fusion tests. It also induced exophoria and caused subjective (visual analogue scales) drowsiness, mental slowness, clumsiness and impaired overall performance. IM proved rather inactive when slightly impairing flicker fusion and digit substitution, and subjectively rendering the subjects clumsier. The combined effects of IM and DZ did not differ from those obtained with DZ alone. Both IM and DZ induced dizziness and their effects in this respect were additive when the drugs were used in combination. It is concluded that single therapeutic doses of indomethacin do not produce major psychomotor effects and do not in this respect increase the effects of diazepam. However, the feeling of dizziness, a side-effect common to both these drugs, may be additive when the drugs are used in combination.