Researchers uncover causal evidence that cannabis legalization reduces problematic consumption

Allowing adults to legally purchase cannabis from pharmacies may help reduce problematic use—especially among those who use other drugs—according to new research from Switzerland. The first-of-its-kind randomized controlled trial, published in the journal Addiction, found that legal access to regulated cannabis products was associated with a modest reduction in cannabis misuse over six months compared to continued use through the illegal market.

The study was designed to address a pressing question facing many countries: does regulating cannabis access reduce its harms? While recreational cannabis laws are becoming more common, particularly in parts of Europe and North America, most prior research on their impact has been observational. That means it can be hard to know whether changes in cannabis use or mental health outcomes are actually caused by the laws themselves or by other factors, such as regional differences or shifts in social attitudes.

To overcome these challenges, researchers in Basel, Switzerland, conducted a randomized controlled trial—the first of its kind—to compare legal cannabis access with continued use of the illegal market. The trial was conducted under Switzerland’s unique legal framework that permits pilot studies of legal recreational cannabis. The goal was to examine how a public health-oriented system of regulated cannabis access, including product labeling and optional counseling, might affect users’ consumption and mental well-being.

The study enrolled 378 adults who regularly used cannabis and lived in the canton of Basel-Stadt. Participants were randomly assigned to one of two groups: half were given access to legal cannabis through nine local pharmacies, while the other half continued obtaining cannabis through illegal sources. The legal cannabis was strictly regulated, with quality control standards, capped tetrahydrocannabinol (THC) content, THC-dependent pricing, and access to educational materials and counseling. Participants were followed over six months and completed online surveys about their cannabis use and mental health.

The primary outcome was cannabis misuse, measured using the Cannabis Use Disorders Identification Test-Revised (CUDIT-R), a standard self-report questionnaire. At the start of the study, the average score for both groups was around 11, which is above the threshold typically used to screen for problematic cannabis use. By the six-month mark, participants in the legal access group had a slightly lower average score (10.1) than those in the illegal market group (10.9). Although this difference was not statistically significant by conventional standards (P = 0.052), the researchers note that it was close and consistent across different types of analysis.

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Importantly, when the researchers looked more closely at people who used other drugs in addition to cannabis, the benefit of legal access became clearer. Among these participants, those in the legal cannabis group showed a greater reduction in misuse compared to those in the illegal market group. The difference was nearly two points on the CUDIT-R scale—an amount considered to reflect a meaningful change in behavior. In contrast, no such difference was found among participants who did not use other drugs.

Secondary outcomes included symptoms of depression, anxiety, and psychosis, as well as changes in how much cannabis, alcohol, and other drugs participants used. On these measures, there were no statistically significant differences between the two groups. For example, both groups reported similar levels of depressive and anxiety symptoms after six months, and the total amount of cannabis consumed did not differ meaningfully between legal and illegal users.

About half of the participants in the legal cannabis group also reported continuing to use cannabis from the illegal market. While this crossover reduced the purity of the experimental design, additional analyses showed that excluding these participants did not change the main results.

The researchers also examined whether counseling or access to lower-risk cannabis products played a role in the outcomes. Although formal counseling was voluntary, participants had regular contact with trained pharmacy staff and received informational materials. These interactions may have contributed to the reduced misuse seen among people who also used other drugs, a group that may benefit from increased contact with harm reduction services.

No increase in adverse mental health outcomes was observed in the legal access group. In fact, serious adverse events were rare and evenly distributed between the groups, and none were linked to the cannabis products provided by the study.

While the findings are promising, the authors caution against drawing broad conclusions just yet. The study sample was relatively small, short-term, and not fully representative of the broader population. For example, most participants were men, and individuals with severe mental illness were excluded. Also, many participants continued to use cannabis from illegal sources, suggesting that more work is needed to design legal systems that fully meet users’ needs.

All of the outcomes in this study were based on self-reported data, which can be influenced by participants’ desire to present themselves in a favorable light. And because the study was open-label—participants knew whether they were in the legal or illegal group—this could have affected their responses.

Despite these limitations, the study breaks new ground in evaluating the effects of cannabis regulation under controlled conditions. It offers early evidence that a tightly regulated, health-focused model of legal cannabis access may reduce problematic use among some people, particularly those with more complex drug use patterns. The researchers emphasize that not all legalization models are alike, and outcomes may vary depending on the specific regulatory approach.

Longer-term follow-up is currently underway to examine whether these effects persist or change over time. Future studies will also explore other health outcomes, such as respiratory symptoms, sleep, and physical activity, as well as whether different types of legal cannabis access—like social clubs or dispensaries—produce different results.

The study, “Effects of legal access versus illegal market cannabis on use and mental health: A randomized controlled trial,” was authored by Lavinia Baltes-Flueckiger, Regine Steinauer, Maximilian Meyer, Adrian Guessoum, Oliver Herrmann, Christoph Felix Mosandl, Jens Kronschnabel, Eva-Maria Pichler, Marc Vogel, and Marc Walter.