That is, at least, if you believe the latest research from Imperial College London, University College London, and King's College London, which argues that marijuana smokers who started young and kept it up could have lower motivation:
The researchers found that long-term cannabis users who started young tend to have lower dopamine levels in their brains, and that can translate into a low-energy lifestyle.
The academics actually though they were going to find that these tokers had higher dopamine levels than normal as a result of their reported, occasional "psychotic-like symptoms," which can be associated with high dopamine.
But no: When the researchers used brain imaging to examine these subjects, they found the opposite was true, according to a summary:
[Warning: NSFW language]:
The lowest dopamine levels were seen in users who meet diagnostic criteria for cannabis abuse or dependence ...
The study was published recently in the journal Biological Psychiatry.
Michael Bloomfield of Imperial College says the study might ...
... explain the 'amotivational syndrome' which has been described in cannabis users, but whether such a syndrome exists is controversial.