But the correspondence between these chemicals (like serotonin) and depression is relatively weak. An emerging competitive theory, inspired in part by ketamine’s effectiveness, has it that psychiatric disease is less about chemical imbalance than structural changes in the brain—and that a main cause of these changes is psychological stress. “I really do think stress is to mental illness as cigarettes are to heart disease,” says Gerard Sanacora, the psychiatry professor running the ketamine trial at Yale.

The theory describes stress grinding down individual neurons gradually, as storms do roof shingles. This, in turn, changes the nature of their connections to one another and the structure of the brain. Ketamine, along with some similar molecules, acts to strengthen the neuron against that damage, affecting not just the chemistry of the brain but also its structure.