Cannabis use among patients experiencing a first episode of psychotic illness is linked to a 50% heightened admission risk, as well as longer inpatient stay, the largest study of its kind has revealed.
Cannabis use was also linked to higher numbers of prescriptions for different antipsychotic drugs, the study findings show, suggesting that it may contribute to treatment failure.
Cannabis use has previously been linked to an increased risk of psychotic episodes, but it was not clear if it has any impact on relapse risk in those with long term psychosis, said the researchers in the journal BMJ Open.
They looked at the electronic health records of 2,026 people treated for a first episode of psychosis at one of the largest providers of mental health services in Europe from 2006-13.
They looked at the prevalence of cannabis use recorded within a month of the first visit for early intervention treatment and to track subsequent treatment and outcomes for up to five years afterwards.
Cannabis use was recorded in the records of 46.3% of those using early intervention services within a month of the start of treatment. Use was especially common among single men, aged 16 to 25.
“The findings prompt further study to investigate the mechanisms underlying poor clinical outcomes in people who use cannabis”
Use of the drug was associated with a 50% increase in the frequency of hospital admissions, with an average of 1.8 admissions up to five years after the first service visit, compared with non-users who averaged 1.2 admissions.
In addition, the researchers found it was associated with an increase in the risk of compulsory detention in hospital under the Mental Health Act – 45% of those who used cannabis, compared to 34% of those who did not.
They noted that use of the drug was associated with a significantly longer hospital stay as well, particularly once two years of the monitoring period had elapsed.
Length of stay progressively increased from an average of 21 extra days within three years, to 35 additional days within five years among cannabis users.
Furthermore, cannabis use was associated with a greater likelihood of being treated with clozapine, an antipsychotic used for schizophrenia that is difficult to treat, and a higher number of prescriptions for different antipsychotics.
Many different scripts was indicative of treatment failure, suggested the study authors, and pointed towards a link between cannabis use and increased risk of hospital admission linked to it.
The researchers said their findings “highlight the importance of ascertaining cannabis use in people receiving care for psychotic disorders and prompt further study to investigate the mechanisms underlying poor clinical outcomes in people who use cannabis.”