Chocolate has an effect on the brain similar to opium, a study has shown.
The discovery may explain why the tempting treat is so hard to resist.
A natural brain chemical called enkephalin surged as rats began to eat M&M chocolate sweets. When a drug was used to stimulate the dorsal neostriatum, the brain region releasing the chemical, they more than doubled the number of M&Ms they ate.
Enkephalin is an endorphin with similar properties to opium. In the brain, it binds to molecular "receptors" sensitive to opiate chemicals to reduce pain and produce pleasurable feelings.
The US findings are published in the journal Current Biology.
Previous research has mostly linked the dorsal neostriatum to movement. Recent work has suggested it also plays a role in reward-driven behaviour.
"This means that the brain has more extensive systems to make individuals overconsume rewards than previously thought," said study leader Dr Alexandra DiFeliceantonio, from the University of Michigan.
"The same brain area tested here is active when obese people see foods and when drug addicts see drug scenes. It seems likely that our enkephalin findings in rats mean that this neurotransmitter may drive some forms of over-consumption and addiction in people."
In their paper the scientists concluded: "Opioid circuitry ... could in this way participate in normal motivations and perhaps even in generating intense pathological levels of motivation to over-consume reward in binge eating disorders, drug addiction and related compulsive pursuits."