Cocaine users are often thin because the drug alters their metabolism, a study suggests.
Previously it was widely assumed cocaine use led to weight loss by suppressing appetite.
The discovery has implications for helping people come off the drug. Weight gain can be dramatic and lead to relapse.
Researchers from Cambridge University studied the body composition, diets and eating habits of more than 60 men, half of whom were dependent on cocaine.
Cocaine users showed a preference for fatty foods and carbohydrates, and also displayed patterns of uncontrolled eating.
Despite this, they often experienced weight loss and had less body fat than non-users.
Men taking cocaine also had low levels of the hormone leptin, which inhibits appetite. Low leptin levels combined with high fat consumption should normally lead to weight gain rather than weight loss.
The evidence was published in the journal Appetite.