cannabis psychosis
Cannabis users are more at risk of psychosis relapse.Getty Images

Psychotic patients face worse treatment outcome if they use cannabis, a study claims.

The findings, published in the BMJ Open, show that people who go through a psychotic episode for the first time will relapse more frequently if they use the drug.

Scientists from Kings College London examined the medical records of 2,026 first-time psychotic patients treated in London between 2006 and 2013. They looked at the date when the patients were admitted in hospital for first treatment, and researched any information regarding potential drug abuse.

For the next five years following treatment, they took a note of every hospital readmission for psychosis, and assessed the number of unique anti-psychosis drugs prescribed to each patient.

50% more risk for users

Within one month of the first hospitalisation and treatment, more than 46% of patients had used cannabis at least one time. This was particularly the case amongst single men aged between 16 and 25. The researchers' analysis suggests that cannabis users were then 50% more likely to be readmitted into hospital for another psychosis episode than non-users, in the five years after receiving initial treatment.

Amongst hospitalised psychosis patients, the length of the hospital stay was significantly longer if they has used cannabis. It lasted 35 days in average, compared to 21 days for those who did not use cannabis.

More anti-psychosis drugs

The researchers also discovered that cannabis users were often prescribed more psychosis medicine than their counterparts during the five years of the follow-up, suggesting that first intention drugs failed more often than not to help such patients. In particular, the cannabis users were more frequently given clozapine, a powerful drug commonly used for schizophrenia.

Although they still cannot explain how cannabis affects the brain of psychotic patients and how it may interfere with treatments, the study's authors note that it does lead to a worse outcome. These results are therefore a clear indication that people with psychotic disorders should avoid cannabis.

What is psychosis?

Psychosis is a psychiatric problem which involves hallucinations and delusions. Patients interpret reality differently than those around them.

Psychosis is not a disorder in itself but it is triggered by other mental health problems such as schizophrenia, trauma. depression or bi-polar disorders. People with brain tumours or Parkinson's disease sometimes also go through episodes of psychosis.

Anti-psychotic treatments include both medication and therapies (often cognitive behavioural therapy).

Data on how many people suffer from it in the UK vary greatly from one source to another, but a recent study established that one new case of psychosis is diagnosed for every 2,000 people in England annually.

Other research found that 3 in 100 people will have at least one episode of psychosis at some point in their life.