With medical marijuana legalized in fourteen states and approaching legalization in many others, studying the effects of chronic use is more relevant than ever. A recent study, released this month in PNAS, suggests that marijuana abuse may inhibit reponse to the reward chemical dopamine, and brings to light what may be an increased risk for depression and anxiety associated with chronic abuse of the drug.

The Study

Researchers from several different universities conducted the study on a pool of 48 participants, half of whom “had been smoking a median of about five joints a day, five days a week for 10 years," and were marked as heavy users. The other half of participants were non-users. Both groups of participants were administered the drug methylphenidate, most commonly known as Ritalin, which is used to treat ADD and known for stimulating dopamine production in the brain. The participants then underwent extensive blood tests, PET scans, and interviews in order to monitor the effects of the desired dopamine increase.

The Results

Upon examining tests results, researchers found strongly decreased responses to Ritalin in the marijuana users group, as compared to the non-users group. The control, or non-users group, experienced much more pronounced differences in mood, heart rate, and blood pressure in response to the drug. The control group reported feeling affected by the drug, while the experimental, or users, group did not experience “the cardiovascular, behavioral [or] brain changes usually associated with an increase in dopamine levels." The conducted PET scans backed up the findings, confirming more pronounced physical changes in the brains of the control group in response to the administered Ritalin.

This indicates that prolonged marijuana abuse may inhibit the ability of users to respond to dopamine, the reward system in the brain that is responsible for feelings of motivation. Because of this, they may be more vulnerable to anxiety disorders and depression.

Understanding the Effects of Marijuana on the Brain

A variety of other drugs, such as cocaine, nicotine, alcohol, heroin, and methamphetamines stimulate the release of dopamine in the brain, and therefore inhibit the brain's natural ability to produce dopamine. However, the same link between use and reduced natural dopamine production has not been previously found for marijuana, making it more difficult to understand the effects of long-term use of the drug.

This study helps illuminate some of the interactions between marijuana and dopamine levels, and researchers explained that the reduced responses to dopamine may explain the drug craving, depression, and anxiety symptoms that accompany long-term marijuana abuse. The researchers stated that “marijuana abusers show lower positive and higher negative emotionality scores than controls, which is consistent, on one hand, with lower reward sensitivity and motivation and, on the other hand, with increased stress reactivity and irritability."

However, the results of the study are disputable, and the head author of the study emphasized the need for further research, stating that "moves to legalize marijuana highlight the urgency to investigate effects of chronic marijuana in the human brain."

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Nora D. Volkow, Gene-Jack Wanga, Frank Telanga, Joanna S. Fowler, Jean Logand, Millard Jaynea, Christopher Wonga, Dardo Tomasia. Decreased dopamine brain reactivity in marijuana abusers is associated with negative emotionality and addiction severity. Proceedings of the National Academy of Sciences. DOI:10.1073/pnas.1411228111

Date of original publication:

Updated: September 12, 2019