Study Shows Marijuana's Long-Term Effects On The Brain

The effect of marijuana use depend on age of first use and duration of use, according to a new study.
The effect of marijuana use depend on age of first use and
duration of use, according to a new study. (Image source)
The effects of chronic marijuana use on the brain may depend on age of first use and duration of use, according to researchers. In a paper published in Proceedings of the National Academy of Sciences (PNAS), researchers for the first time comprehensively describe existing abnormalities in brain function and structure of long-term marijuana users with multiple magnetic resonance imaging (MRI) techniques. Findings show chronic marijuana users have smaller brain volume in the orbitofrontal cortex (OFC), a part of the brain commonly associated with addiction, but also increased brain connectivity.

The research team studied 48 adult marijuana users and 62 gender- and age-matched non-users, accounting for potential biases such as gender, age and ethnicity. The authors also controlled for tobacco and alcohol use. On average, the marijuana users who participated in the study consumed the drug three times per day. Cognitive tests show that chronic marijuana users had lower IQ compared to age-and gender-matched controls but the differences do not seem to be related to the brain abnormalities as no direct correlation can be drawn between IQ deficits and OFC volume decrease.
 
Tests reveal that earlier onset of regular marijuana use induces greater structural and functional connectivity. Greatest increases in connectivity appear as an individual begins using marijuana. Findings show severity of use is directly correlated to greater connectivity.
 
Although increased structural wiring declines after six to eight years of continued chronic use, marijuana users continue to display more intense connectivity than healthy non-users, which may explain why chronic, long-term users "seem to be doing just fine" despite smaller OFC brain volumes, the researchers explained.
 
The study offers a preliminary indication that gray matter in the OFC may be more vulnerable than white matter to the effects of delta-9-tetrahydrocannabinol (THC), the main psychoactive ingredient in the cannabis plant. According to the authors, the study provides evidence that chronic marijuana use initiates a complex process that allows neurons to adapt and compensate for smaller gray matter volume, but further studies are needed to determine whether these changes revert back to normal with discontinued marijuana use, whether similar effects are present in occasional marijuana users versus chronic users and whether these effects are indeed a direct result of marijuana use or a predisposing factor.