For people suffering from depression, medication, the most common treatment, is not always a suitable choice. Some depressed patients find the side effects of antidepressants—which include a “drugged” feeling, insomnia, and sexual dysfunction—to be intolerable. Other patients believe their mental disorders are not serious enough to warrant drug intervention. Still others categorically reject the idea of using medication to alleviate mental problems. Of the patients who refrain from taking antidepressants, many opt for non-drug therapies such as meditation, exercise, or dietary supplements. One such supplement that is gaining widespread attention is the herb St. John’s wort.

St. John’s wort, a yellow-flowered plant whose Latin name is Hypericum perforatum, has been used therapeutically for over 2,000 years. Today, the herb is Germany’s leading treatment for depression—and its popularity...