Marijuana’s Effects on Health

A recent article on “Adverse Health Effects of Marijuana Use” in the New England Journal of Medicine talked about the current science on marijuana. They outlined a number of risks.


  • Long-term marijuana use can lead to addiction.
    • Approximately 9% of those who experiment with marijuana will become addicted.
    • The number goes up to 1 in 6 for those who start using marijuana as teenagers and about 25-50% among those who smoke marijuana daily.
  • “Cannabis withdrawal syndrome” makes it harder to quit and contributes to relapse. Symptoms include irritability, trouble sleeping, craving and anxiety.
  • If you use cannabis as a teen, you are 2-4 times more likely to have dependence symptoms after 2 years of use.

Brain Development

  • Adults who smoked cannabis regularly as teens have impaired neural connectivity in specific brain regions.
    • The precuneus, which regulates alertness and self-conscious awareness
    • The fimbria, which is important to learning and memory
  • Reduced connectivity in several areas of the brain.
  • Imaging studies in cannabis users show decreased activity in prefrontal regions and reduced volume in the hippocampus.
    • These negative effects are more prominent if use starts in the teen and young adult years.
  • There is a link between frequent marijuana use in the teen years and decreased IQ.

The “Gateway Drug” effect

  • When marijuana exposure happens at a young age, it appears to affect the reward systems in the brain. This may explain why individuals who used marijuana in adolescence and earlier have increased susceptibility to drug use and addiction later in life.

Relation to Mental Illness

  • Marijuana has been linked with psychoses, including schizophrenia, especially among those with a pre-existing genetic vulnerability to those diseases.
  • Heavy use, use of especially potent marijuana and exposure at a younger age can advance the time of a first psychotic episode by 2-6 years.
  • It is not clear if using marijuana actually increases the risk of psychosis.

Lifetime Achievement

  • Studies are unclear about whether deficits in brain development as a result of marijuana use can be reversed.
    • Early marijuana use is associated with impaired school performance and an increased risk of dropping out of school, but it is not clear how much environment impacts these negative consequences.
  • Heavy marijuana use has been linked to lower income, unemployment, criminal behavior and lower satisfaction with life.

Risk of Driving Accidents

  • Higher blood THC levels result in poor performance in controlled driving simulation studies.
  • Drivers testing positive for THC were 3-7 times more likely to be responsible for a motor vehicle accident as those who had not used alcohol or other drugs before driving.
  • The risk of accident appears to be much greater when marijuana is combined with alcohol.

Risk of Cancer and Other Health Effects

  • There is not a clear link between marijuana smoking and lung cancer, in part because many individuals smoke both tobacco and marijuana.
  • Marijuana smoking is associated with inflammation of the large airways and other respiratory effects, but there do not seem to be long-term negative effects.
  • Marijuana use has been associated with vascular conditions that increase the risk of heart attack and stroke. The links are not clear but seem to be connected to the direct effect cannabinoids have on vasoactive compounds and target receptors in blood vessels.

Gaps in Knowledge

  • More knowledge and understanding is needed to harness the medical benefits of marijuana while reducing or eliminating the risks of the drug.
  • For example, it is clear that marijuana helps reduce wasting syndrome in AIDS patients, but use of the drug may worsen HIV-associated cognitive deficits.
  • There are also areas (like lung cancer) where more information is needed to separate correlation from causation.

What We Do Know

  • The strongest case for negative effects is in addiction, diminished lifetime achievement, motor vehicle accidents, and chronic bronchitis.
  • Confidence is lower for abnormal brain development, progression to use of other drugs, schitzophrenia, and depression and anxiety.
  • Confidence for a link between smoking marijuana and lung cancer is low.

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