Cannabis Use Depletes Dopamine Long Term
In the same way cocaine and heroin are well known to deplete your domamine* long term. it seems long term cannabis use can do the same leading to depression.
*Dopamine is the chemicals in your brain that make you feel good. When you take cocaine and other such drugs like molly the dopamine is very quickly send from your brain into your body to give you the high that many chase.
Drug Researchers in the UK have seen evidence that the levels of dopamine in a part of the brain called the Stratium were significantly reduced in cannabis users when compared to non users.
Worryingly the levels were much worse in those that were heavy users from a young age.
You can read more here.