Opponents argue that among other things it causes anxiety, mood swings and depression.
While advocates counter that it’s beneficial for migraines and helps people cope with a range of diseases, from Parkinson’s to Crohn’s.
Now, a new study published in the journal Molecular Psychiatry has found that heavy use of the drug can result in damaging the area of the brain called the striatum. Specifically, it found that users suffered a “compromised dopamine system”.
Dopamine is chemical that is integral to the brain's reward system which is linked to learning and memory.
These side effects had been found previously in users of cocaine and heroin, but evidence from marijuana use was missing until now.
The study involved 11 adults between the ages of 21 and 40 who were highly dependent on cannabis and 12 matched healthy controls.
On average, the cannabis group started using at age 16 and became dependent on cannabis by age 20.
In the month prior to the study, nearly all users in this study smoked marijuana daily.
In contrast to the controls, the cannabis users had significantly lower dopamine release in the striatum, in addition to other areas of the brain which are significant for learning.
"We don't know whether decreased dopamine was a pre-existing condition or the result of heavy cannabis use," said Dr Anissa Abi-Dargham, professor of psychiatry (in radiology) at Columbia University Medical Center (CUMC) and a lead author of the paper.
"But the bottom line is that long-term, heavy cannabis use may impair the dopaminergic system, which could have a variety of negative effects on learning and behaviour.
"In light of the more widespread acceptance and use of marijuana, especially by young people, we believe it is important to look more closely at the potentially addictive effects of cannabis on key regions of the brain."
Jeffrey Lieberman, MD, Chair of Psychiatry at CUMC added that "these findings add to the growing body of research demonstrating the potentially adverse effects of cannabis, particularly in youth, at the same time that government policies and laws are increasing access and use".
There are few drugs which provoke more public debate than marijuana.