Marijuana affects cannabinoid receptors which are found in brain regions that influence learning, memory, appetite, coordination and pleasure. Affecting those receptors is how marijuana use produces the effects it has on users.
What researchers do not fully understand is what effect that marijuana can have on the brain when someone uses the drug regularly over a long period of time. MRI imaging studies show that there are differences between the brains of marijuana users and non users.
According to the National Institute on Drug Abuse, what scientists do not know is what the differences seen on the MRI images mean and how long the difference last, if someone quits using marijuana.
Research into the effects of marijuana on the brain has been hampered by the fact that most people who use marijuana also drink alcohol, which can have its own negative effects on the brain.
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National Institute on Drug Abuse. "Marijuana." DrugFacts Updated January 2014
National Institute on Drug Abuse. "Want to Know More?- Some FAQs about Marijuana." Marijuana: Facts for Teens Updated October 2013
National Institute on Drug Abuse. "Marijuana." Research Report Series Updated July 2012
The Partnership at DrugFree.org. "Marijuana." Drug Guide. Accessed April 2014.