Visual evoked potentials as a predictor of reported side effects during a trial of zimelidine vs. placebo in chronic pain patients.

Before a double-blind controlled study of zimelidine, a new, rather specific inhibitor of serotonin uptake, in patients with chronic pain, visual evoked potentials (EPs) to stimuli of varying intensities were recorded. During the trial the blood levels of zimelidine and its active metabolite norzimelidine were estimated. Side effects were rated by a physician before and after the trial. No significant difference in frequency of side effects could be found between the zimelidine and the placebo groups. Furthermore, there was no significant relationship between the blood levels of the active drug or its metabolite and the frequency of side effects. Instead, a significant relationship was found in the total group between the frequency of side effects and the tendency to react with an increase in maximum amplitude of the EP when stimulus intensity was increased (i.e., augmenting). The same trend was clear both in the zimelidine group and in the placebo group. Thus the augmenting/reducing response in visual EPs seems to be a predictor of the side effects reported irrespective of the drugs given.