Cannabis use among patients with first episode psychosis is associated with substantially worse clinical outcomes, according to a large study led by King's College London.
Patients with psychosis who had a history of cannabis use were more likely to be admitted to hospital, to require compulsory admission, and to spend an extra 35 days in hospital in the five years after their first episode, the study found. Cannabis use was also linked to prescription of several different antipsychotic drugs during the follow-up period, suggesting treatment failure.
The five-year study included 2,026 people with first episode psychosis who were accepted by an early intervention service in the South London and Maudsley NHS Foundation Trust, one of the largest providers of mental health services in Europe. Cannabis use was noted in the records of 46.3 per cent of patients using the intervention services within a month of starting treatment and was particularly common in single men aged 16 to 25.
Its use was associated with a 50 per cent higher frequency of hospital admissions. Users had an average of 1.8 admissions in the five years after the first service visit, compared with an average of 1.2 admissions among non-users in the same period.
The drug was also associated with increased odds of compulsory detention under the Mental Health Act: 45 per cent in those who used cannabis, compared with 34 per cent in those who did not.
The study also linked cannabis use to a greater number of days spent in hospital, significant from year two onwards. The length of stay increased from an average of 21 extra days within three years to 35 extra days within five years among cannabis users.
And cannabis use was linked to an increased likelihood of being treated with clozapine and to a higher number of prescriptions for different antipsychotics. The number of unique antipsychotics prescribed ranged from 0 to 11.
The research team, led by Dr Rashmi Patel from the Institute of Psychiatry, Psychology & Neuroscience (IoPPN), said that they could not establish whether patients were resistant to a given antipsychotic or whether their prescription had been changed to a new drug because of relapse or side effects. However, they said that this suggested an association between cannabis use and increased risk of hospital admission linked to treatment failure.
They added that, as the study was observational, no firm conclusions could be drawn about cause and effect. But they concluded that 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, and strategies to reduce associated harms.'