Marijuana (Cannabinoids)

Effects of cannabis on the brain

The effects of cannabis are mediated via cannabinoid CB1 receptors in the brain.

Researchers at the National Institute of Mental Health, in Bethesda, Maryland, measured the effects of chronic daily cannabis smokers on these receptors.

First, the details.

  • 30 male chronic daily cannabis smokers were monitored at an inpatient research unit for about 4 weeks.
    • Positron emission tomography (PET) images were taken of cannabis smokers on the day following admission and after about 4 weeks of abstinence.
      • A PET scan shows how organs and tissues inside the body are functioning.
  • 20 healthy men with less than 10 lifetime cannabis exposure were scanned once.
  • Before the scan, each participant took a radioligand that binds to the cannabis receptor.
  • Arterial blood was sampled during PET scans to estimate receptor binding.

And, the results.

  • The number of receptors decreased about 20% in the brains of cannabis smokers compared to healthy control subjects with limited exposure to cannabis.
  • The changes correlated with the number of years they smoked.
  • Among cannabis smokers given a second PET scan after about a month of abstinence there was a marked increase in receptor activity in those areas that had been decreased at the outset of the study.

The bottom line?

The results indicate that while chronic cannabis smoking causes down regulation of the CB1 receptors. However, the good news is that the damage is reversible with abstinence.

The authors concluded, “Chronic daily cannabis smoking is associated with reversible down regulation of cortical cannabinoid CB1 receptors in human brain. Cannabinoid CB1 receptor down regulation could be among neuroadaptations that promote cannabis dependence in human brain.”


6/8/11 20:17 JR

Hi, I’m JR

John Russo, Jr., PharmD, is president of The MedCom Resource, Inc. Previously, he was senior vice president of medical communications at, a complementary and alternative medicine website.