The Food and Drug Administration’s approval last month of a depression treatment based on ketamine generated headlines, in part, because the drug represents a completely new approach for dealing with a condition the World Health Organisation has labeled the leading cause of disability worldwide. The FDA’s approval marks the first genuinely new type of psychiatric drug—for any condition—to be brought to market in more than 30 years. Despite this excitement, researchers still don’t know exactly how ketamine exerts its effects. A leading theory proposes that it stimulates regrowth of synapses (connections between neurons), effectively rewiring the brain. Researchers have seen these effects in animals’ brains, but the exact details and timing are elusive.