Patients' responses to treatment with antidepressants vary greatly. The largest study on the effectiveness of antidepressants to date suggested that roughly one third of patients will recover fully given a long enough trial, one third will improve substantially, and one third will fail to respond.1 A subset in all three groups will have adverse effects such as sexual dysfunction, insomnia, nausea, and weight gain. Side effects are rarely dangerous, but they cause many patients to discontinue treatment, often after a single prescription.2

Similar concerns exist for many drugs, not just antidepressants. What makes antidepressants especially frustrating for clinicians and patients is the lack of factors to predict how individual patients will respond to a given treatment. We know that one third of patients will have minimal improvement, but we have few data to guide selection of alternative treatments. Indeed, while the psychiatric literature abounds with reports of clinical predictors, the findings are rarely replicated.3 One of the only replicated predictors, that atypical …