A study led by Jussi  Hirvonen, MD, PhD, in a collaborative effort between the National Institute of Health, and the National Institute on Drug Abuse, revealed that chronic marijuana use decreases the amount of CB1 receptors (Cannabinoid Receptors) in the brain.

In this study researchers recruited 30 daily cannabis smokers and monitored them in a closed inpatient facility for approximately 4 weeks. Subjects were injected with radiogland, 18-F-FMPEP-d2, which is a brain transmitter analog that binds to CB1 receptors in the brain. Using a positron emission tomography (PET) to image the brain scientist noticed that in chronic marijuana smokers there were 20% less CB1 receptors when compared to human controls with limited marijuana exposure in their lifetime. A correlation found was between the number of years the subject had smoked and decrease in receptor activity.

Of the 30 original patients, 14 of the subjects underwent a second PET scan after a month of abstinence. Receptor activity increased in the areas that showed markedly decreased receptor activity earlier in the study, which was an indication that damage was reversible with abstinence.