Cannabis use did not worsen cognition in first-episode schizophrenia, but methamphetamine did | Research

The takeaway

Over two years of treatment, methamphetamine use predicted less cognitive improvement in schizophrenia patients, while cannabis use did not.

Clinicians and researchers studying how substance use affects cognitive outcomes in psychotic disorders.

Cannabis use did not predict cognitive decline over 24 months

What the researchers found

In 81 patients with first-episode schizophrenia spectrum disorders treated over 24 months, positive methamphetamine tests predicted less cognitive improvement, but positive cannabis tests did not.

Why it matters

This study helps disentangle the cognitive effects of different substances in people with schizophrenia, a population where substance use is common and cognitive outcomes are critical for functioning.

The numbers in context

81 patients and 100 controls followed for 24 months. Patients improved cognitively with treatment but remained significantly below controls throughout. Methamphetamine, not cannabis, predicted less cognitive improvement.

How the study worked

Longitudinal cohort study of 81 patients treated with flupenthixol decanoate over 24 months. Cognition assessed with the MATRICS battery at four time points; urine testing for cannabis and methamphetamine at six time points. Compared to 100 matched controls.

What this study cannot tell us

Substance use was measured by urine testing, which has a limited detection window. The study was observational, so it cannot establish causation. Sample size was moderate.

How to read the evidence

Moderate: longitudinal design with standardized assessments and matched controls, but observational with moderate sample size.

When this study was published

Published in 2022.

The bigger picture

While both cannabis and methamphetamine are commonly used by people with psychotic disorders, their cognitive effects appear to differ substantially. This distinction matters for clinical decision-making.

Questions still open

Does cannabis truly have no cognitive impact in schizophrenia, or is the effect too small for this sample to detect? Would results differ with heavier cannabis use? Does cannabis type or potency matter?

Common questions

Did patients with schizophrenia improve cognitively over time?

Yes, patients showed cognitive improvement with treatment, but their scores remained significantly below those of healthy controls at all time points.

Why might methamphetamine affect cognition differently than cannabis?

Methamphetamine is a potent dopamine-releasing agent with well-documented neurotoxic effects, while cannabis acts through the endocannabinoid system. Their distinct pharmacological profiles may explain the different cognitive impacts.

Does this mean cannabis is safe for people with schizophrenia?

This study only examined cognitive effects. Cannabis use is associated with other risks in psychotic disorders, including symptom worsening and relapse.

Read the original research

The associations of cannabis and methamphetamine use with cognitive performance over the first 2 years of treatment in schizophrenia spectrum disorders.

Early intervention in psychiatry, 16(11), 1230-1239

View on PubMed ↗Publisher’s paper ↗

Citation

Scheffler, Freda; Phahladira, Lebogang; Hendrikse, Chanellé B; du Plessis, Stefan; Asmal, Laila; Luckhoff, Hilmar K; Smit, Anna Margaretha; Olivier, M Riaan; Emsley, Robin. (2022). The associations of cannabis and methamphetamine use with cognitive performance over the first 2 years of treatment in schizophrenia spectrum disorders.. Early intervention in psychiatry, 16(11), 1230-1239. https://doi.org/10.1111/eip.13272