Sanofi's potential blockbuster obesity and anti-smoking drug Rimonabant has its roots in efforts to understand why smoking cannabis tends to make people feel insatiably hungry. Scientists reasoned that if taking the drug gave people the so-called munchies, then identifying and blocking the brain pathways responsible might stop others overeating.

It works by binding to and blocking a receptor protein found on the surface of brain cells. These receptors are set up to receive chemical molecules acting as messengers, and when they do, this triggers a chemical reaction that sends an instruction to the brain. The scientists found that chemicals called canna-binoids, which occur naturally in cannabis, bound to these receptors and made the cells issue an instruction to eat.

The body also produces several cannabinoids of its own, which bind to the same receptor and issue the same instruction. Rimonabant's trick is that it looks similar enough to these endogenous messenger molecules to bind the receptor, but different enough not to cause the "eat" message to be passed on. It then tends to hang around on the receptor protein, effectively acting as a protective cap that stops the genuine cannabinoid signal molecules from binding.

Results from trials have shown encouraging weight loss. Obese patients given the highest doses of the drug lost an average of 4kg each in 16 weeks compared to a control group. Scientists have found that it also seems to suppress a nicotine addict's urge to smoke and others are investigating whether it could be used to combat alcoholism.

Steve Alexander, a neuroscientist who works on cannabinoid receptors at Nottingham University says drugs like Rimonabant tap into pathways of the brain associated with pleasure. "The brain processes you undergo as you eat nice food are very similar to what you do when you smoke cannabis or inject heroin.