Scientists believe they have discovered why some users of the illegal drug, cannabis, lack motivation to work or pursue their normal interests.

They have found that long-term users of the drug produce less dopamine, a chemical in the brain that is linked to motivation.

They scanned the brains of 19 people who regularly used the drug and 19 people who did not use it and found that dopamine levels in the part of the brain called the striatum were lower in heavy users and those who began taking the drug from a younger age.

The scientists noted that all of the users of the drug had experienced psychotic-like symptoms while smoking it, such as having bizarre thoughts or experiencing strange sensations.

As a result, the team from various colleges in London, expected to find higher levels of dopamine, as increased production of this chemical in the body has been linked to psychosis. However, the opposite was true.

Furthermore, the lowest levels of dopamine were found in cannabis users who were classed as dependent on the drug.

All of the drug-users in the study began smoking cannabis between the ages of 12 and 18.

Previous studies have found that people who use cannabis have an increased risk of suffering mental health problems related to repeated episodes of psychosis, such as schizophrenia.

"It has been assumed that cannabis increases the risk of schizophrenia by inducing the same effects on the dopamine system that we see in schizophrenia, but this hasn't been studied in active cannabis users until now.

"The results weren't what we expected, but they tie in with previous research on addiction, which has found that substance abusers - people who are dependent on cocaine or amphetamine, for example - have altered dopamine systems," explained lead scientist, Dr Michael Bloomfield, of Imperial College London.

He noted that while the study had only focused on cannabis users who had experienced psychotic-like events while taking the drug, the scientists believe that 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'.

"It could also explain the ‘amotivational syndrome', which has been described in cannabis users, but whether such a syndrome exists is controversial," Dr Bloomfield added.

Details of these findings are published in the journal, Biological Psychiatry.