Marijuana users like to tout that abusing their drug of choice has fewer negative consequences than most other narcotics, but new research out of the Brookhaven National Laboratory, New York University Langone Medical Center and the National Institute on Alcohol Abuse and Alcoholism now suggests marijuana is not as innocuous as once believed, reports Phys.org.

Researchers found that when marijuana abusers were given methylphenidate, or Ritalin, a drug that stimulates dopamine production in the brain, the subjects failed to exhibit the brain and behavioral changes typical of elevated dopamine levels. The study suggests that people who use marijuana excessively may have developed problems with the reward circuitry in their brains.

Previous studies have given conflicting reports about marijuana use and its effect on the brain's dopamine production system. Some have suggested that the drug stimulates dopamine (a neurotransmitter associated with feelings of pleasure), while others have found no link at all. For this new study, researchers bypassed the question of whether marijuana use does or does not stimulate dopamine production, and instead focused on how the brains of marijuana abusers responded when elevated dopamine levels were given.

The study provided Ritalin to 24 excessive marijuana users, each of whom smoked a median of about five joints a day, five days a week, for 10 years. A control group of equal size was also provided with Ritalin. Researchers then compared the two groups' reactions to the heightened levels of dopamine in their systems. Members of the control group experienced greater increases in heart rate and diastolic blood pressure than the marijuana abusers did. The control group also reported feeling more high, restless, anxious and affected by the drug in general. PET scans confirmed differences in brain activation between the two groups. 

Researchers noted that these results only show a correlation between excessive marijuana use and blunted dopamine response. It's still unclear whether excessive marijuana use is actually causing damage to the brain's reward circuitry, or whether people who already have damaged reward circuitry just happen to gravitate more toward using marijuana. Even so, the study does seem to indicate a link between excessive marijuana use and this kind of brain damage.

The takeaway is that marijuana abusers could find it harder to experience pleasure than most people do. Although the study only looked at heavy abusers, it's a wake-up call to marijuana enthusiasts who believe that the drug has few to no negative side-effects, even when used excessively.

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