Advocates of marijuana point to study after study that say cannabis is not dangerous, that it has medicinal purposes, and is safer for people than alcohol and tobacco. It'll be interesting to hear what they have to say about a new study that finds long-term use of marijuana can lead to increased risk of developing hallucinations, delusions and psychosis.
The study out of Australia asked nearly 3,100 people averaging about 20 years of age about their marijuana use. They found that almost 18 percent reported using the drug for three or fewer years, about 16 percent for four to five years, and just over 14 percent for six or more years.
Among those questioned, 65 had been diagnosed with a "non-affective psychosis" such as schizophrenia, and 233 had at least one positive item for hallucination on a diagnostic interview conducted for the study.
Researchers concluded there was an association between length of marijuana use and mental health. Dr. John McGrath wrote:
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"Compared with those who had never used cannabis, young adults who had six or more years since first use of cannabis [i.e., who commenced use when around 15 years or younger] were twice as likely to develop a non-affective psychosis and were four times as likely to have high scores on the Peters et al Delusions Inventory [a measure of delusion]. There was a 'dose-response' relationship between the variables of interest: the longer the duration since the first cannabis use, the higher the risk of psychosis-related outcomes."
But researchers admit the link is complicated. For example, it's not clear if pre-existing psychosis led people to use marijuana, or if the marijuana use came first: They wrote:
"This demonstrates the complexity of the relationship: those individuals who were vulnerable to psychosis [i.e., those who had isolated psychotic symptoms] were more likely to commence cannabis use, which could then subsequently contribute to an increased risk of conversion to a non-affective psychotic disorder."
The study appears online March 1 and in the May issue of the Archives of General Psychiatry.