Avoiding use of high-potency cannabis could prevent around 24% of psychosis cases, according to the researchers.
The findings of the study, led by Dr. Marta Di Forti of the Institute of Psychiatry, Psychology and Neuroscience at King's College London in the UK, add to increasing evidence that suggests a link between cannabis use and mental health.
In December 2013, for example, Medical News Today reported on a study associating heavy marijuana use with abnormal brain changes similar to those found in patients with schizophrenia, while a more recent study revealed how marijuana use can cause paranoia.
For this latest research, Dr. Di Forti and her team assessed the effects of cannabis use among 780 individuals aged 18-65.
Of these, 410 presented at the South London Maudsley National Health Service (NHS) Foundation Trust in London, UK, between 2005 and 2011 reporting a first episode of psychosis. The remaining 360 participants were controls from the same area of London.
Psychosis is a term used to describe symptoms of mental illness, such as hallucinations and delusions. These symptoms often occur alongside mental health disorders, including schizophrenia, bipolar disorder and depression. In 2012, around 43.7 million Americans aged 18 and older had some form of mental illness.
Avoiding use of high-potency cannabis 'could prevent 24% of psychosis cases'
From their analysis, the researchers found that participants who used a form of high-potency skunk-like cannabis were around three times more likely to develop psychosis, compared with those who had never used cannabis.
What is more, this risk increased with regular use of the drug; participants who used the high-potency skunk-like cannabis daily were around five times more likely to develop psychosis, compared with those who had never used cannabis.
The researchers note that they found no association between the use of hash - another potent form of cannabis - and increased risk of psychosis. The team says this may be because the high-potency skunk-like cannabis had higher levels of delta-9-tetrahydrocannabinol (THC) than hash. "Use of cannabis with a high concentration of THC might have a more detrimental effect on mental health than use of a weaker form," they add.
Based on their findings, the researchers calculate that around 24% of psychosis cases could be avoided if people were to stop using this high-potency cannabis.
Senior author Sir Robin Murray, professor of psychiatric research at King's College London, adds:
"It is now well known that use of cannabis increases the risk of psychosis. However, skeptics still claim that this is not an important cause of schizophrenia-like psychosis. This paper suggests that we could prevent almost one quarter of cases of psychosis if no one smoked high potency cannabis."
Dr. Di Forti says their findings also suggest that the risk of psychosis among cannabis users is dependent on the frequency of use and the potency of the cannabis, and these are factors that should be considered by doctors.
"When a GP or psychiatrist asks if a patient uses cannabis it's not helpful; it's like asking whether someone drinks," says Dr. Di Forti. "As with alcohol, the relevant questions are how often and what type of cannabis. This gives more information about whether the user is at risk of mental health problems; awareness needs to increase for this to happen."
The researchers say one possible limitation of their study is that they did not have data on the number of joints cannabis users smoked each day and the grams that they used. "However, because we collected information about use over a period of years and not about present use, the reliability of such detailed information would probably have been confounded by recall bias to a greater extent than was the general description of pattern of use that we obtained," they add.
MNT recently reported on a study associating marijuana use with excessive daytime sleepiness in some adolescents.