How Does Marijuana Affect Health?

Marijuana contains cannabinoids which bind to receptors in the endocannabinoid system.  Two of these cannabinoids are delta-9 tetrahydrocannabinol (commonly known as THC) and cannabidiol.

The THC/cannabidiol ratio

THC is primarily responsible for the psychoactive effects of marijuana, binding to receptors in the brain that are responsible for cognition, memory, reward, pain perception, and motor coordination. THC causes the “high” -- the sense of euphoria, relaxation, and altered senses—users get from marijuana.2  

Cannabidiol is the other main cannabinoid in marijuana. Cannabidiol regulates the body’s response to other cannabinoids and in general, has a moderating influence on the effects of THC. Cannabidiol reduces THC’s psychoactivity and reduces the craving for THC and the likelihood of developing psychotic symptoms.  The relative amounts of THC and cannabidiol that a user receives vary from plant to plant and by the form that the user consumes.

Today’s marijuana is much stronger than a generation ago

There’s been a marked increase in the concentrations of THC in marijuana over the last few decades, while the cannibidiol concentration has been decreasing. In 1989, THC concentration in marijuana was about 2%.  By 2008, the THC concentration had increased to 14%.  Meanwhile, the concentration of cannabidiol decreased from 0.24% in 1996 to 0.08% in 2008.4  This increase in average THC content as well as THC/cannabinidiol ratio has significant implications for rates of adverse effects including psychosis and addiction.1 


  1. Niesink RJ, et al. Does cannabidiol protect against adverse psychological effects of THC? Frontiers in Psychiatry 2013. Volume 4 Article 130 1-8
  2. Hall W, et al. Adverse health effects of non-medical cannabis use. Lancet. 2009 Oct 17;374(9698):1383-91
  3. Devinsky O, et al. Cannabidiol: Pharmacology and potential therapeutic role in epilepsy and other neuropsychiatric disorders. Epilepsia. 2014: 1-12
  4. Burgdorf JR, Kilmer B, Pacula RL. Heterogeneity in the composition of marijuana seized in California. Drug Alcohol Depend.2011;117:59-61


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