This animation addresses the question: Why do people lose control over their cocaine use? Researchers monitored the activity of two types of neurons in mice: “urge” neurons, which promote feelings of reward and repeating behaviors that have produced rewards, and “control” neurons, which dampen those feelings and inhibit behavior. Cocaine shifted the balance of activity strongly to the “urge” neurons. When mice received the drug for the first time, the balance quickly returned to normal; but when they received the drug after already having had it many times, the “urge” neurons continued to predominate for much longer. The researchers suggest that the same dynamic occurs when people use cocaine repeatedly, so that early episodic drug use evolves over time into a perpetual strong urge to seek the drug. (See also Why Take a Drug That No Longer Gives Pleasure?