Thirty-seven depressed patients over the age of 55 were treated for 5-7
weeks with either nortriptyline, a tricyclic antidepressant, or phenelzine,
a monoamine oxidase (MAO) inhibitor. Patients' platelet MAO activity was
measured following a drug washout period before treatment. Patients with
higher MAO activity had a better response to treatment, regardless of which
drug was used.