Legal, But Not Trial-Friendly: Marijuana in Clinical Research

Washington helped make history in 2012 when it became one of the first states in the US to make marijuana legal for adults for personal use. Today, we often hear this question from people interested in clinical trials: “if marijuana is legal, why can’t I use it during a clinical trial”? This is a great question. The answer depends on the study, but it usually comes down to three things: safety, good science, and how studies are designed.

First, safety is a top priority. Some doctors in Washington have recently shared some concerns about mental health problems linked to marijuana use, stating that they are seeing patients with health concerns two to three times per month. The physicians mentioned addiction, anxiety, hallucinations, and severe nausea or vomiting that lead to dehydration. In another survey from Canada, about one out of three people who used marijuana reported at least one health issue in the past year. These included panic attacks, feeling faint, heart problems, or nausea and vomiting . Marijuana can also affect heart rate, thinking, and coordination. These are things that researchers often measure in clinical trials. If marijuana causes similar side effects as the study drug, it can be hard to tell what is causing the problem. That’s why some studies do not allow it: it’s about keeping people safe, not judging anyone.

Second, clinical trials need clear and reliable results. Most studies are testing one new treatment at a time. Researchers try to limit anything else that could affect the results. Marijuana can change mood, memory, and brain function. If someone uses marijuana during a study, it may be hard to know if changes are caused by the study drug or something else. This is especially important in mental health studies, where changes in mood and thinking are the main thing being studied.

Finally, studies are carefully designed to keep things as consistent as possible. Researchers want everyone in the study to have a similar experience, except for the treatment being tested. Marijuana can be very different from person to person. It can vary in strength, ingredients, and how it is used (such as smoking or eating it). It can also stay in the body for a long time. This makes it hard to measure and control, which can affect the study results.

Even though marijuana is legal in Washington, it can still make clinical trials harder to run and understand. Not all studies ban marijuana, but many do for the reasons above. The most important thing is to be honest with your study team about any substances you use. This helps keep you safe and helps researchers get accurate results. As we learn more about marijuana, its role in healthcare and research may change. But at Northwest Clinical Research Center, one thing stays the same: we always put patient safety first and work hard to produce results people can trust.

Written by: Kimberlee Wheeler

Sources:

1. Marijuana Policy Project. Washington. Available at: https://www.mpp.org/states/washington/
2. KOMO News Staff. Survey: More than half of Washington clinicians cite mental-health risks from cannabis use. KOMO News. March 30, 2026. Available at: https://www.msn.com/en-us/health/mindandbody/survey-more-than-half-of-washington-clinicians-cite-mental-health-risks-from-cannabis-use/ar-AA1ZKDzm
3. Marquette A, Iraniparast M, Hammond D. Adverse outcomes of cannabis use in Canada, before and after legalisation of non-medical cannabis: cross-sectional analysis of the International Cannabis Policy Study. BMJ Open. 2024;14(1):e077908. Available at: https://bmjopen.bmj.com/content/14/1/e077908
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Burggren AC, et al. Cannabis effects on brain structure, function, and cognition. 2019. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC7027431/
de Souza Queiroz AP, et al. Cognitive Effects of Cannabis Use: A Review. 2025. Available at: https://pmc.ncbi.nlm.nih.gov
Health Canada. Health effects of cannabis. Government of Canada. March 25, 2024. Available at: https://www.canada.ca/en/health-canada/services/drugs-medication/cannabis/health-effects/effects.html
5. Burggren AC, et al. Cannabis effects on brain structure, function, and cognition. 2019.
Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC7027431/