1. The respiratory and psychomotor effects of two benzodiazepines used mainly as anticonvulsants were compared in healthy volunteers, using a double‐blind placebo controlled design. 2. Clobazam (10 and 20 mg) produced significantly fewer psychomotor side effects than clonazepam (0.5 and 1 mg). Neither drug at either dose affected the ventilatory response to CO2. 3. Although clonazepam produced significant effects on psychomotor performance, these did not correlate with plasma drug concentration. 4. Our studies provide further evidence that at the doses chosen clobazam is considerably less sedating than clonazepam. Further investigation is required into the tolerance profile of both drugs in patients.

1. The influence of pharmaceutical formulation on the plasma drug concentration‐time curve and the psychomotor responses to 400 mg carbamazepine has been assessed in 12 healthy male volunteers; three formulations and placebo were compared in a randomised, blind, crossover study. 2. The plasma concentration of carbamazepine rose to a maximum of 3‐7 mg l‐1 by 2‐3 h after administration of the liquid suspension. Conventional and controlled release tablet formulations gave lower peaks at about 8 and 32 h, respectively. From 32 h onwards the plasma concentrations from the three formulations were indistinguishable. 3. Significant impairment of psychomotor function was observed after the liquid suspension only; subjective sedation was significant at 1 and 2 h and the critical flicker fusion frequency threshold was lowered at 1‐8 h. Digit‐symbol substitution, choice reaction time and body sway gave less conclusive evidence of impairment. 4. The results do not support the hypothesis that a psychomotor effect from carbamazepine is a threshold phenomenon with a critical plasma drug concentration at about 8 mg l‐1. 5. A second hypothesis that rate of rise of plasma carbamazepine concentration has an important influence on psychomotor effect fits the observations. This interpretation is tentative since the use of a fixed dose of carbamazepine meant that differences due to rate of rise of drug concentration were confounded with differences due to peak height.

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