Smoking cannabis/indian hemp/marijuana makes people less motivated or lazy, say scientists.
Long-term use of the drug destroys dopamine, the feel-good chemical in the brain that inspires a spirit of get-up-and-go (spirit of motivation).
Previous research has suggested taking marijuana can lead to individuals becoming withdrawn, lethargic and apathetic. However, a new study has shown levels of dopamine in a part of the brain called the striatum - found towards the side of the brain and involved in motivation - were lower in regular cannabis users.
Dr Michael Bloomfield, of Imperial College London, said: 'Dopamine is involved in telling the brain when something exciting is about to happen - be it sex, drugs or rock ‘n roll.
'Our findings explain why cannabis has a tendency to make people sit around doing nothing.
PET (positron emission tomography) scans on the brains of 38 people - 19 regular cannabis users and 19 non-users - found less dopamine in those who smoked more and those who began taking the drug at a younger age.
The researchers said this could explain why some cannabis users seem to lack motivation to work or pursue their normal interests.
The cannabis users had their 1st experience with the drug between the ages of 12 and 18 and the researchers believe the drug could be the cause of the difference in dopamine levels.
The lowest dopamine levels were seen in users who met diagnostic criteria for cannabis abuse or dependence - raising the possibility this measure could provide a marker of addiction severity.
Previous research has shown cannabis users have a higher risk of mental illnesses that involve repeated episodes of psychosis such as schizophrenia.
Dr Bloomfield said: 'Cannabis is an illegal drug and there is mounting evidence the idea of it being a harmless herb is not true.
'Cannabis has negative effects on the brain and it’s important people can make an informed decision.'
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