Cannabis legalization made it harder to keep patients enrolled in medical cannabis research | Research

The takeaway

After Canada legalized recreational cannabis, retention in a prospective medical cannabis study dropped significantly, with patients who could access cannabis elsewhere less likely to stay enrolled.

Cannabis researchers, clinical trial designers, and anyone interested in the challenges of studying a substance that is simultaneously legal and medically authorized.

72% drop in retention odds after recreational legalization

What the researchers found

Legalization of non-medical cannabis was associated with significantly lower odds of study retention (AOR 0.28, 95% CI 0.18-0.41). Baseline opioid use also predicted dropout, while higher psychological health scores and anti-seizure medication use predicted staying in the study.

Why it matters

High dropout rates threaten the validity of cannabis research. Understanding why patients leave studies is critical for designing future trials that produce reliable evidence on medical cannabis benefits and harms.

The numbers in context

1,011 participants; 287 patient-years of data; 72% retained at 3 months; 41.4% at 6 months; post-legalization retention AOR 0.28; opioid use AOR 0.62; anti-seizure medication AOR 1.91

How the study worked

The Tilray Observational Patient Study followed 1,011 Canadian medical cannabis patients across multiple sites from 2016 to 2019. Kaplan-Meier survival analysis and logistic regressions assessed factors associated with retention at 3 and 6 months.

What this study cannot tell us

Convenience sample limits generalizability. Self-reported cannabis use data. Industry-sponsored study (Tilray). Cannot fully separate legalization effects from other temporal factors.

How to read the evidence

Multi-site prospective study with survival analysis, though convenience sampling and industry sponsorship limit generalizability.

When this study was published

Published in 2021 using data from 2016-2019.

The bigger picture

This study reveals an ironic challenge: legalizing cannabis for recreational use may actually undermine the medical research needed to understand cannabis as medicine, as patients no longer need to stay in studies to access the product.

Questions still open

How can researchers design studies that retain participants when cannabis is freely available? Does dropout bias systematically overestimate or underestimate medical cannabis benefits? Would compensating participants adequately solve the retention problem?

Common questions

Why did patients drop out after legalization?

The study suggests patients no longer needed to stay enrolled in a medical cannabis study to access cannabis, since they could buy it legally through recreational dispensaries or the illicit market.

What kind of patients stayed in the study?

Patients with higher psychological health scores and those using anti-seizure medications were more likely to remain enrolled, possibly because they were more motivated by medical outcomes rather than just access.

Read the original research

The impact of non-medical cannabis legalization and other exposures on retention in longitudinal cannabis research: a survival analysis of a prospective study of Canadian medical cannabis patients.

Journal of cannabis research, 3(1), 34

View on PubMed ↗Publisher’s paper ↗

Citation

Lucas, Philippe; Boyd, Susan; Milloy, M-J; Walsh, Zach. (2021). The impact of non-medical cannabis legalization and other exposures on retention in longitudinal cannabis research: a survival analysis of a prospective study of Canadian medical cannabis patients.. Journal of cannabis research, 3(1), 34. https://doi.org/10.1186/s42238-021-00089-7