Patients who meet screening criteria enter a 2-week washout period (or, for fluoxetine, a 4-week washout period) during which no psychotropic medication is permitted. Participants are then randomly assigned to St. John's Wort, citalopram, or placebo for 12 weeks. Participants who respond to treatment by Week 12 continue to take their originally assigned double-blind medication for up to 26 weeks. At Week 12, nonresponders to placebo are crossed over to 1 of the 2 active treatments. Patients who remain nonresponders to that active treatment are crossed over to the alternative active treatment, with the investigator maintaining blind status. Participants have study visits at screening, during the washout period, at baseline, and every 2 weeks thereafter for the course of the study. At the screening visit, Week 12, and Week 20, patients undergo a complete blood count with differential and other routine laboratory tests.