Zammit and colleagues from the University of Bristol, Imperial College and Cambridge University examined 35 studies that tracked tens of thousands of people for periods ranging from one year to 27 years to examine the effect of marijuana on mental health.
They looked for psychotic illnesses as well as cognitive disorders including delusions and hallucinations, bipolar disorder, depression, anxiety, neuroses and suicidal tendencies. They found that people who used marijuana had roughly a 40 percent higher chance of developing a psychotic disorder later in life. The overall risk remains very low.
"The strongest case is that there are consistencies across all of the studies," and that the link was seen only with psychoses - not anxiety, depression or other mental health problems, he said.