Imaging scans of the brains of people who smoke marijuana daily show a decrease in the number of receptors involved in pleasure, appetite, pain tolerance and many other key mental and bodily functions, a new study shows.

The study, presented at the Society of Nuclear Medicine Annual Meeting, included 30 chronic daily marijuana smokers. They were monitored at an inpatient facility for four weeks. Their brains were scanned using positron emission tomography (PET). The study found certain receptors in the brains of marijuana smokers were decreased by about 20 percent compared with people who had a limited lifetime exposure to marijuana, also known as cannabis. “With this study, we were able to show for the first time that people who abuse cannabis have abnormalities of the cannabinoid receptors in the brain,” lead researcher Dr. Jussi Hirvonen said in a news release.

The researchers then re-scanned the brains of 14 of the marijuana users who had abstained from smoking marijuana for one month and found a marked increase in the brain receptors compared with the beginning of the study, HealthDay reports. The researchers say the findings suggest that the damaging effects of marijuana use are reversible.

The study was a collaboration between the U.S. National Institute of Mental Health and U.S. National Institute on Drug Abuse.