Besides making a bongo drum sound inexplicably magical and enhancing a person’s ability to talk nonsense for extended periods of time, generations of cannabis smokers will recognise the “munchies” as one of the drug’s most reliable side-effects.

Now scientists have shown that the insatiable urge to eat after smoking is caused by cannabinoids hijacking brain cells that normally suppress appetite. The study suggests that cannabis causes the brain to produce a different set of chemicals that transform the feeling of fullness into a hunger that is never quite satisfied.

Scientists believe the findings, which illuminate a previously unknown aspect of the brain’s feeding circuitry, could help design new drugs that would boost or suppress appetite at will.

Tamas Horvath, who led the work at Yale University, said: “By observing how the appetite centre of the brain responds to marijuana, we were able to see what drives the hunger brought about by cannabis and how that same mechanism that normally turns off feeding becomes a driver of eating. It’s like pressing a car’s brakes and accelerating instead.”

Scientists have previously shown that activating a cannabinoid receptor in the brain, called CB1R, tends to trigger an increased desire to eat. Until now, however, it was not known which bits of the brain’s appetite circuitry were involved. The latest study tested this in mice, by injecting cannabinoids into the brain and monitoring which neurons were activated in response.

The research, published in Nature, unexpectedly showed that activity was boosted in a group of nerve cells called POMC (pro-opiomelanocortin) neurons, which normally produce feelings of satiety.

“We were surprised to find that the neurons we thought were responsible for shutting down eating, were suddenly being activated and promoting hunger, even when you are full,” said Horvath. “It fools the brain’s central feeding system.”

Further investigation showed that the cannabis “subverted” these neurons, causing them to release hunger stimulating chemicals rather than appetite suppressing chemicals, explaining how the drug can produce a sudden urgent desire for a packet of Doritos or a bowl of Coco Pops.

The scientists speculate that cannabinoids “flips the switch” on the neurons by binding to tiny energy-generating organelles inside the cells, called mitochondria, in addition to receptors on the neurons’ surface.



Intriguingly, people who smoke cannabis regularly do not tend to gain weight – if anything they are less likely to be obese. It is not clear whether this is because the “munchies” effect wears off in people who smoke regularly, or whether habitual smokers eat less when not under the influence of the drug.

The scientists are now exploring whether the brain circuitry that produces the “munchies” is also central to the feeling of being “high”.

Previously, Japanese researchers have shown that cannabis appears to interact with taste receptors to enhance the sweet taste in foods, thus boosting certain cravings. Other work has shown that mice given THC (tetrahydrocannabinol), one of the active ingredients in cannabis, had an enhanced sense of smell and an overactive “reward” system, which provides hints to why some people find eating especially gratifying when under the influence.