A new study has shown that chocolate has an effect on the brain similar to opium.
A natural brain chemical called enkephalin surged when 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.
Previous research has mostly linked the dorsal neostriatum to movement. Recent work has suggested it also plays a role in reward-driven behaviour.
The US findings are published in the journal Current Biology.