Cocaine users are often thin because the drug drastically alters their metabolism, a study suggests.
Previously it was widely assumed that cocaine use led to weight loss by suppressing appetite.
The discovery has implications for helping people come off the drug. Weight gain during recovery can be dramatic and lead to relapse.
Researchers 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 significantly 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.
Taken together the evidence, published in the journal Appetite, indicates that the metabolism of cocaine users is significantly altered.
Study leader Dr Karen Ersche, from Cambridge University, said: "Our findings challenge the widely held assumptions that cocaine use leads to weight loss through appetite suppression. Rather, they suggest a profound metabolic alteration that needs to be taken into account during treatment.
"We were surprised how little body fat the cocaine users had in light of their reported consumption of fatty food. It seems that regular cocaine abuse directly interferes with metabolic processes and thereby reduces body fat. This imbalance between fat intake and fat storage may also explain why these individuals gain so much weight when they stop using cocaine but not in cocaine users."
She added: "For most people weight gain is unpleasant but for people in recovery, who can gain several stones, this weight gain goes far beyond an aesthetic concern but involves both psychological and physiological problems. The stress caused by this conspicuous body change can also contribute to relapse.
"It is therefore important that we better understand the effects of cocaine on eating behaviour and body weight to best support drug users on their road to recovery."
