India, Oct. 31 -- Antidepressants are the usual first treatment for depression, and they act slowly. A patient waits four to six weeks to learn whether a drug is helping. If it is not, the doctor changes the dose or the drug, and the wait begins again. Once two different antidepressants have failed, both taken at a full dose for long enough, psychiatrists call the illness treatment-resistant depression. Regulators such as the European Medicines Agency use the same two-drug threshold. The odds of recovery fall with every further drug: in STAR*D, the largest US study of patients moving through successive treatments, about 37% recovered on the first drug and 31% on the second, but only about 13% on the third or fourth. In the US, researchers...