cannabis use

A recent study determines the relationship between continued cannabis use on medication adherence and patient outcomes. The researchers investigate whether the poor outcome in psychosis patients using cannabis is due to its negative effect on anti-psychotic medications.


Psychosis is a serious mental condition in which a person has problems in perceiving reality. They may experience a range symptoms, including hallucinations (hearing voices or seeing things that are not perceived by others) or delusions (false beliefs). They may also act inappropriately or speak incoherently.Psychosis has a wide range of causes –it may be a symptom of a mental illness such as schizophrenia or bipolar disorder, or it may occur following alcohol abuse or use of drugs such as cannabis or LSD. It can also be related to an underlying brain abnormality such as a tumor, stroke, Parkinson’s disease, or Alzheimer’s disease.

Relapse during the first few years after an initial episode of psychosis is an important predictor of a patient’s long-term outcome. Identifying risk factors that could lead to relapse is key. Although there are many factors that can influence relapses, continued cannabis use following the onset of psychosis and not taking antipsychotic medication regularly are both seen consistently in studies as significant risks. It is important to understand the relationship between these factors in order to help patients with psychosis. Researchers at King’s College London explored whether some of the adverse effects of continued cannabis use on risk of relapse are due to its association with poor medication adherence. They recently reported their findings in The Lancet Psychiatry.

Patients who had an initial episode of psychosis not related to an underlying brain abnormality between April 2002 and July 2013 were approached for a follow-up interview assessment about their cannabis use and medication adherence in the first 2 years following their diagnosis.Relapse data and medication adherence during this time were also collected from their clinical notes.

Of 397 patients approached, 245 agreed to take part in the study. Ninety-one of these patients (37%) had experienced a relapse in the two years following their initial psychosis episode. Continued cannabis use predicted poor outcome — including relapse risk and number, length, and severity of relapses. In a further analysis, the researchers estimated that between 20-36% of the adverse effects of continued cannabis use on outcome in psychosis might be due to its effects on medication adherence.

This study indicates that patients who continue to use cannabis following their first episode of psychosis are less likely to take their medications as prescribed and, that these patients are more likely to relapse.  The researchers suggest that interventions aimed at improving medication adherence could help to lessen the harmful effects of cannabis use in patients with psychosis.

Written by Julie McShane, Medical Writer


Schoeler T, Petros N, Di Forta M, et al. Poor medication adherence and risk of relapse associated with continued cannabis use in patients with first-episode psychosis: a prospective study. The Lancet Psychiatry. Published online July 10, 2017.

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