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Is there a link between marijuana use and mental illness?

Several studies have linked marijuana use to increased risk for mental illnesses, including psychosis (schizophrenia), depression, and anxiety, but whether and to what extent it actually causes these conditions is not always easy to determine.19 The amount of drug used, the age at first use, and genetic vulnerability have all been shown to influence this relationship. The strongest evidence to date concerns the link between marijuana use and psychotic disorders in those with a preexisting genetic or other vulnerability.42 Recent research (see AKT1 Gene Variations and Psychosis) has found that marijuana users who carry a specific variant of the AKT1 gene, which codes for an enzyme that affects dopamine signaling in the striatum, are at increased risk of developing psychosis. The striatum is an area of the brain that becomes activated and flooded with dopamine when certain stimuli are present. One study found that the risk for those with this variant was seven times higher for daily marijuana users compared with infrequent- or non-users.43

Bar graph comparing AKT1 gene variants and their carriers' risk for psychosisWhether adolescent marijuana use can contribute to developing psychosis later in adulthood appears to depend on whether a person already has a genetically based vulnerability to the disorder. The AKT1 gene governs an enzyme that affects brain signaling involving the neurotransmitter dopamine. Altered dopamine signaling is known to be involved in schizophrenia. AKT1 can take one of three forms in a specific region of the gene implicated in susceptibility to schizophrenia: T/T, C/T, and C/C. Daily users of marijuana (green bars) with the C/C variant have a seven times higher risk of developing psychosis than infrequent marijuana users or nonusers. The risk for psychosis among those with the T/T variant was unaffected by whether they used marijuana.

Source: Di Forti et al. Biol Psychiatry. 2012.

Another study found an increased risk of psychosis among adults who had used marijuana in adolescence and also carried a specific variant of the gene for catechol-O-methyltransferase (COMT), an enzyme that degrades neurotransmitters such as dopamine and norepinephrine.44 (see Genetic variation in COMT influences the harmful effects of abused drugs). Marijuana use has also been shown to worsen the course of illness in patients who already have schizophrenia. As mentioned previously, marijuana can also produce a brief psychotic reaction in non-schizophrenic users, especially at high doses, although this fades as the drug wears off.

Bar graph showing how the influence of adolescent marijuana use on adult psychosis is affected by COMT genetic variation The influence of adolescent marijuana use on adult psychosis is affected by genetic variables. This figure shows that variations in a gene can affect the likelihood of developing psychosis in adulthood, following exposure to cannabis in adolescence. The COMT gene governs an enzyme that breaks down dopamine, a brain chemical involved in schizophrenia. It comes in two forms: "Met" and "Val." Individuals with one or two copies of the Val variant have a higher risk of developing schizophrenic-type disorders if they used cannabis during adolescence (dark bars). Those with only the Met variant were unaffected by cannabis use.

Source: Caspi et al. Biol Psychiatry. 2005.

Other, less consistent associations have been reported between marijuana use and depression, anxiety, suicidal thoughts among teens, and personality disorders. Marijuana has also been associated with an amotivational syndrome, defined as a diminished or absent drive to engage in typically rewarding activities. Because of the role of the endocannabinoid system in regulating mood and reward, it is logical to hypothesize the existence of such a link underpinned by brain changes, but more research is needed to confirm and better understand it.

Adverse Consequences of Marijuana Use

Student with hands on his head

Acute (present during intoxication)

  • Impaired short-term memory
  • Impaired attention, judgment, and other cognitive functions
  • Impaired coordination and balance
  • Increased heart rate
  • Anxiety, paranoia
  • Psychosis (uncommon)

Persistent (lasting longer than intoxication, but may not be permanent)

  • Impaired learning and coordination
  • Sleep problems

Long-term (cumulative effects of repeated use)

  • Potential for addiction
  • Potential loss of IQ
  • Increased risk of chronic cough, bronchitis
  • Increased risk of schizophrenia in vulnerable people*
  • Potentially increased risk of anxiety, depression, and amotivational syndrome*

* These are often reported co-occurring symptoms/disorders with chronic marijuana use. However, research has not yet determined whether marijuana is causal or just associated with these mental problems.


This page was last updated September 2015

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