Young people who use marijuana for six years or more are twice as likely to have psychotic episodes, hallucinations or delusions than people who have never used the drug. This new discovery adds weight to earlier research which connected psychosis with marijuana – especially in its strongest form called "skunk" – and will add to the raging debate about the drug.
According to an estimate from the United Nations, approximately 190 million people use marijuana. This is about 4 percent of the adult population. John McGrath of the Queensland Brain Institute in Australia studied more than 3,801 men and women born between 1981 and 1984. McGrath followed up with them after 21 years and inquired about their marijuana use. McGrath also inquired about any psychotic episodes. About 18 percent of the people reported marijuana use for three or less years, 16 percent for four to five years, and 14 percent for six or more years.
McGrath wrote that "compared with those who had never used cannabis, young adults who had six or more years since first use of cannabis were twice as likely to develop a non-affective psychosis (such as schizophrenia)." These people were also four times as likely to have high scores in clinical tests of delusion. McGrath also found that the longer the time since the first marijuana use, the higher the risk of psychosis-related symptoms.
A study last year by British researchers said that people who smoke skunk, a potent form of marijuana, are almost seven times more likely to develop mental illnesses like schizophrenia than those who smoke ‘hash" or cannabis resin.
The British study was the first one to look specifically at skunk. Skunk has higher amounts of the psychoactive ingredient THC which can produce psychotic symptoms such as hallucinations, delusions and paranoia.
