Researchers at the University of California San Diego have completed a landmark survey of more than 5,000 Californians to study the effects of Proposition 64: The Adult Use of Marijuana Act, which made recreational cannabis use legal for adults 21 years of age and older across the state in 2016. The results of the survey, titled Impact 64, reveal that while many Californians are using cannabis regularly for both recreational and medicinal purposes, there are significant gaps in knowledge around cannabis use and its impacts. Initial results of the survey were recently published in the journal Cannabis and Cannabinoid Research .
"Our study provides the first comprehensive understanding of cannabis use patterns in California since the implementation of Proposition 64," said Linda Hill, M.D., M.P.H., principal investigator of Impact 64, distinguished professor at the UC San Diego Herbert Wertheim School of Public Health and Human Longevity Science, clinical professor at the School of Medicine and affiliated faculty at the Qualcomm Institute.
"It's clear that a large portion of the population is using cannabis, but there are significant gaps in knowledge regarding laws and safety, and physicians are rarely consulted for advice on use. We need more targeted education initiatives for both cannabis users and medical providers."
The new study found:
- 37% of California adults reported cannabis use in the past three months. 58% consumed cannabis daily, and 40% of users reported very frequent use (multiple times a day).
- 77% reported obtaining cannabis products from dispensaries, which the majority (94%) perceived as licensed.
- Regardless of self-categorization as a "medicinal" or "recreational user," many participants reported improvements in emotional (82%), mental (81%) and physical (62%) health from cannabis.
- Although 78% felt comfortable discussing cannabis with their primary doctor, only 66% of participants reported that their physicians are aware of their cannabis use, and few users (15%) turn to medical providers for information and guidance about cannabis.
The passage of Proposition 64 in 2016 has driven rapid growth in California's cannabis industry. The California Department of Cannabis Control (DCC) reports that there were more than $4.2 billion in cannabis sales across the state in 2024. As cannabis use becomes more mainstream with legalization in additional states, concerns have also increased about the risks it could pose to public health, such as impaired driving.
Impact 64, supported by the DCC, sought to evaluate cannabis use among adults in California in the post-Proposition 64 era, including frequency of use, sources, attitudes, relationship with health care providers, and sources of information about cannabis. By collaborating closely with Quester, a market research firm, the team was able to gather both qualitative and quantitative data with an AI-powered survey. Partnering with Quester also helped ensure that the study population was a representative sample of Californians, including native Spanish speakers.
"These methods allowed us to collect more in-depth data than is possible in a typical survey-based study," said study co-author Renee Dell'Acqua, M.P.H, research program manager for the Herbert Wertheim School of Public Health's Transportation Research and Education for Driving Safety (TREDS) center. "The result is an incredibly rich picture of how adults are using cannabis in California that can be used to inform future policy and public health initiatives."
The study also assessed knowledge of the laws and behaviors related to cannabis use and driving, and these results are still pending publication. Future studies are also needed to explore cannabis-related attitudes, knowledge and practices among medical providers.
The research highlights the need for education and guidance on safe cannabis use practices, particularly given the prevalence of use in California.
"There is still much to be learned regarding the evolving world of legalized cannabis, including how it's being used, by who, and the level of risks it may pose to public safety," said Thomas Marcotte, Ph.D. co-principal investigator of Impact 64, professor of psychiatry and co-director of the Center for Medicinal Cannabis Research at UC San Diego School of Medicine. "The risk-benefit ratio for cannabis use is complex and warrants further study."
Additional co-authors of the study include Daniel Ageze, Jill Rybar, and Sara Baird at UC San Diego Herbert Wertheim School of Public Health and Human Longevity Science and Alice Gold, Tom Shaughnessy and Ilene Lanin-Kettering at Quester.
This study was funded by the California Department of Cannabis Control (Grant #: 65334).
