Verbal Memory Improved in Schizophrenia Patients Who Stopped Using Cannabis for 28 Days | Research

The takeaway

Cannabis-dependent patients with schizophrenia who achieved 28 days of abstinence showed significant improvements in verbal learning and memory, with a large effect size.

Readers interested in how cannabis use affects cognition in people with schizophrenia.

Verbal memory improved with a large effect size (d = 1.07) after 28 days of cannabis abstinence in schizophrenia

What the researchers found

Nineteen cannabis-dependent patients with schizophrenia and 20 cannabis-dependent controls without psychiatric illness attempted 28 days of cannabis abstinence. Abstinence rates were similar between groups (42% of patients, 55% of controls).

Among the schizophrenia patients who achieved abstinence, verbal learning and memory (measured by the Hopkins Verbal Learning Test-Revised) improved significantly over the 28 days (p < 0.03), with a large effect size (d = 1.07). Controls who abstained showed a smaller improvement (d = 0.66).

The finding that schizophrenia patients showed greater cognitive recovery than controls suggests that cannabis may be disproportionately impairing cognitive function in people with schizophrenia, and that these effects are at least partially reversible with abstinence.

Why it matters

Cannabis use is extremely common among people with schizophrenia, and there has been debate about whether it helps or harms cognitive function in this population. This study provides prospective evidence that abstinence leads to measurable cognitive improvement, particularly in verbal memory, a domain critical for daily functioning and treatment engagement.

The numbers in context

19 schizophrenia patients and 20 controls. 42% of patients achieved abstinence vs. 55% of controls (not significantly different, p = 0.53). Verbal memory improvement in schizophrenia abstainers: d = 1.07 (large effect). Controls: d = 0.66 (medium effect). Abstinence verified by urinalysis at day 28.

How the study worked

Prospective 28-day cannabis abstinence study with cognitive testing at days 0, 14, and 28. Abstinence was incentivized through contingency reinforcement and verified by twice-weekly urinalysis (THC-COOH < 20 ng/ml at day 28). A comprehensive neuropsychological battery assessed multiple cognitive domains. Clinical symptoms were monitored weekly.

What this study cannot tell us

Very small sample size (fewer than 10 abstainers per group after dropout). Only male participants. The contingency reinforcement paradigm may not translate to real-world clinical settings. Less than half of participants achieved the full 28 days of abstinence. The study could not determine whether longer abstinence would produce further improvement.

How to read the evidence

Preliminary evidence from a small prospective study with high dropout rates.

When this study was published

Published in 2017. One of few controlled abstinence studies in dual-diagnosis patients.

The bigger picture

The dual diagnosis of schizophrenia and cannabis dependence is one of the most challenging clinical scenarios in psychiatry. This study suggests that addressing cannabis use could meaningfully improve cognitive outcomes in these patients. The large effect size for verbal memory improvement is clinically significant, as memory deficits are among the most functionally disabling aspects of schizophrenia.

Questions still open

Do cognitive improvements continue beyond 28 days of abstinence? Would female patients show similar patterns? Can effective cannabis cessation interventions be integrated into standard schizophrenia treatment? Does the cognitive improvement translate to better functional outcomes?

Common questions

Does cannabis make schizophrenia symptoms worse?

This study specifically looked at cognition rather than psychotic symptoms. It found that stopping cannabis improved verbal memory in schizophrenia patients, suggesting cannabis was contributing to cognitive impairment. Effects on other symptoms were assessed weekly but the abstract focused on cognitive outcomes.

Why is it so hard for people with schizophrenia to stop using cannabis?

Less than half of the schizophrenia patients in this study achieved 28 days of abstinence, even with financial incentives. Cannabis use in schizophrenia is common and persistent, possibly because patients use it to self-medicate negative symptoms or medication side effects, though this study did not explore reasons for use.

Read the original research

Effects of Extended Cannabis Abstinence on Cognitive Outcomes in Cannabis Dependent Patients with Schizophrenia vs Non-Psychiatric Controls.

Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 42(11), 2259-2271

View on PubMed ↗Publisher’s paper ↗

Citation

Rabin, Rachel A; Barr, Mera S; Goodman, Michelle S; Herman, Yarissa; Zakzanis, Konstantine K; Kish, Stephen J; Kiang, Michael; Remington, Gary; George, Tony P. (2017). Effects of Extended Cannabis Abstinence on Cognitive Outcomes in Cannabis Dependent Patients with Schizophrenia vs Non-Psychiatric Controls.. Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, 42(11), 2259-2271. https://doi.org/10.1038/npp.2017.85