Smoking cannabis really does make people less motivated, 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.
Previous research has suggested taking marijuana can lead to individuals becoming withdrawn, lethargic and apathetic.
Now 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.
‘The results weren’t what we expected but tie in with previous research on addiction which has found substance abusers have altered dopamine systems.
‘Although we only looked at cannabis users who have had psychotic-like experiences while using the drug we think the findings would apply to cannabis users in general since we didn’t see a stronger effect in the subjects who have more psychotic-like symptoms. This needs to be tested though.
‘It could also explain the “amotivational syndrome” which has been described in cannabis users but whether such a syndrome exists is controversial.’
Some claim there is no such condition saying a person who fits this description could in fact be showing signs of depression or chronic intoxication.
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 in the study published in Biological Psychiatry had all experienced psychotic-like symptoms while smoking the drug such as strange sensations or having feelings of paranoia.
The researchers expected their dopamine production might be higher since this has been linked with psychosis – but instead found the opposite.
The cannabis users had their first 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: ‘It’s been assumed cannabis increases the risk of schizophrenia by inducing the same effects on the dopamine system we see in schizophrenia but this hasn’t been studied in active cannabis users until now.’
Other studies have looked at dopamine release in former cannabis users and not seen differences with people who haven’t taken the drug – suggesting the effects seen in this study are likely to be reversible.
Dr Bloomfield said: ‘Cannabis is an illegal drug and there is mounting evidence the idea of it being a harmless herb is not true.
‘When people stop taking cannabis it seems the brain can slowly go back to producing pretty normal levels of dopamine.
‘Cannabis has effects on the brain and it’s important people can make an informed decision.’
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