A new study suggests marijuana blunts the brain’s reaction to dopamine, making users less responsive to the chemical responsible for feelings of reward and pleasure.

In the study published Monday in the journal Proceedings of the National Academy of Sciences, the researchers studied the brains of 24 marijuana abusers — that is, people who smoked multiple times a day — and how they reacted to methylphenidate, a stimulant often used to treat ADHD and narcolepsy. Using personality tests and brain imaging, the researchers found the pot users had blunted behavioral, cardiovascular and brain responses to methylphenidate compared with control participants. Marijuana abusers scored lower on tests of positive emotional activity and higher on negative emotional reactions.

The researchers believe that pot not only dampens the brains’ dopamine reaction to stimulants but also influences the area of the brain involved in reward processing. The participants had lower reward sensitivity, higher levels of irritability, and likely more depression and anxiety.

The researchers conclude that the way pot interferes with the brain may contribute to drug cravings. And that a “high” is really the opposite in the brain.

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