Liam Davenport

April 06, 2016

FLORENCE, Italy — Cannabis use increases the risk for nonadherence to antipsychotic medications, suggesting that discouraging cannabis use may improve not only treatment adherence but also the prognosis of psychosis patients, say United Kingdom investigators.

The researchers found that cannabis use more than doubles the risk for nonadherence to antipsychotics and that continued use of the drug increases the risk for nonadherence more than fivefold.

Enrico Foglia, from the Institute of Psychiatry, Psychology and Neuroscience at King's College London, United Kingdom, told Medscape Medical News that this is the first meta-analysis of the relationship between cannabis use and antipsychotic medication nonadherence.

The findings were presented here at the Schizophrenia International Research Society (SIRS) 2016 Biennial Meeting.

Identify Vulnerable Patients

The strength of the association, which was consistent across the studies, did not come as a surprise, inasmuch as it is known that antipsychotic nonadherence is "a big problem, and it's very prevalent, and I think it's quite intuitive" that cannabis use would reduce adherence, Foglia said.

Enrico Foglia

His interpretation of the results is that it is not necessarily that cannabis itself "induces nonadherence but the fact that people who smoke cannabis will also be more prone to not take medication." Moreover, "discontinuing cannabis may be a sign that they say, 'I want to do something about myself, and I'm also going to take medication.' "

He believes that it is "really important" to identify at-risk individuals and to dissuade them from smoking cannabis, particularly insofar as previous studies have shown that people with high levels of schizotypy "are most vulnerable to cannabis use, and they experience more psychoticlike symptoms when smoking cannabis."

To examine the relationship between cannabis use and adherence to antipsychotics, the researchers conducted a systematic database search in accordance with the MOOSE guidelines.

Random effects models were used to determine pooled odds ratios for the risk for nonadherence to antipsychotics in cannabis users vs nonusers at baseline. In addition, nonusers were compared with continued cannabis users and former users at follow-up, and former users were compared with continued cannabis users.

A total of 15 observational, longitudinal prospective or retrospective studies, involving a total of 3678 patients, were identified for further analysis. Sufficient data were available for 11 of these studies to be included in the pooled effect-size analysis.

The team calculated that there was a significantly increased risk for antipsychotic medication nonadherence in cannabis users at baseline in comparison with nonusers, at an odds ratio of 2.46 in a total of 3055 patients.

At follow-up, continued cannabis users had an increased risk for nonadherence compared with both nonusers and former users, at odds ratios of 5.79 (n = 175) and 5.50 (n = 192), respectively. However, there was no increased risk for nonadherence in former users compared with nonusers, at an odds ratio of 1.12 (n = 187).

On metaregression and subgroup analyses, the researchers found that the results were not explained by differences in medication type, diagnosis, illness severity at baseline, study quality, follow-up duration, age, sex, distribution, and time lag between cannabis use and adherence measurements.

Discussing the findings, Foglia said that there is currently a big debate as to the question of causality between cannabis smoking and psychosis: "Is cannabis causing psychosis independently, or are people who are more prone to psychosis also more drawn to use cannabis?

"We still don't know. There are studies that suggest a causal relationship...but other studies found the opposite."

He added that it "will be interesting to explore the reasons why people start using cannabis." He noted that some studies that have focused on the pleasurable effects of smoking cannabis have found that the people who discontinue use of the drug experience fewer pleasurable effects and more negative effects than those who continue.

This, he said, suggests that there may be genetic components to cannabis smoking that play a role in continuation.

The investigators report no relevant financial relationships.

Schizophrenia International Research Society (SIRS) 2016 Biennial Meeting: Poster T131, presented April 5, 2016.

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