Relationship between response to phenelzine and MAO inhibition in a clinical trial of phenelzine, amitriptyline and placebo.

This report examines the hypothesis that for phenelzine to be more effective than placebo it is necessary to achieve at least 80% inhibition of platelet MAO activity. This hypothesis was examined in the context of a double-blind comparison of phenelzine, amitriptyline and placebo in depressed patients. When phenelzine became significantly more effective than placebo at 4 weeks, the average MAO inhibition was 85%. By the 5th week, with MAO inhibition greater than 90%, phenelzine was significantly more effective than amitriptyline. A highly significant correlation was noted between improvement and MAO inhibition within the phenelzine group.