Health Effects of Cannabis Use: The Current Evidence on Benefits & Risks

Gillian Schauer, PhD, MPH

For this blog, we talked about the health effects of cannabis use with Dr. Gillian Schauer, a national cannabis expert and senior consultant to federal and state agencies on cannabis policy, surveillance, and research. The health effects of cannabis and cannabinoids are complex, and we know far less than we do about other substances. Dr. Schauer helps us sort fact from fiction, and calls out areas where we don’t yet have definitive answers.

We’ve provided some highlights from Dr. Schauer’s response, with her full report on our King County Youth Marijuana Prevention and Education Program’s webpage. Please see Dr. Schauer’s full report for complete and specific descriptions of evidence behind the information in this blog.


With cannabis being legal in Washington, in ten other U.S. states, and the District of Columbia, you’ve probably heard a lot lately about both the good and the potential harm that can come from using cannabis. The truth is, we don’t know as much as we’d like to about any of the effects of cannabis. You can find a scientific study to support almost anything you want to say about the health effects of cannabis, and anecdotes (both good and bad) abound. This report digs into the science, summarizing what we know and what we don’t know about the benefits and risks cannabis can have to health.

The best sources of information on the health effects of cannabis will usually be reviews, where scientists have looked at all of the studies that have been conducted in a certain area, assessed their quality (e.g., sample, design, analyses), and conducted some analyses to pool or summarize results across the high quality studies. For this report, we are relying heavily on four such published reviews: one by the National Academies of Sciences, one by the World Health Organization, and two by the Colorado Department of Public Health and Environment.

It’s important to note that what we know about the health effects of cannabis is almost all by association. This means that we really cannot say that cannabis “causes” any of the things we talk about in this report, yet. Studies have not been designed in a way that establishes causality. It will be important to advance science so we have a better understanding in the future of whether the effects we describe are caused by cannabis use, or just occur at higher rates in cannabis users due to other factors. In addition, some of the research we review in this report has not been conducted using the entire cannabis plant, but rather isolated components of the plant (or cannabinoids).

Potential therapeutic effects associated with cannabis use
Cannabis has a long history of use as a medicinal plant. While clinicians may recommend cannabis for a broad array of medicinal uses, and states may authorize a variety of medical conditions for cannabis use, the science is still fairly limited on the medical benefits of cannabis. There are five areas for which we have modest to substantial evidence of therapeutic effects from cannabis: relieving chemotherapy-induced nausea and vomiting, treating of seizures associated with two rare forms of childhood-onset epilepsy, symptoms from multiple sclerosis-related spasticity, treating chronic pain in adults, and improving short-term sleep outcomes in individuals with specific sleep disturbances. Read Dr. Schauer’s full report for the evidence on therapeutic effects. She additionally covers areas in which evidence is limited or insufficient to draw conclusions about therapeutic benefits.

Potential harmful effects associated with cannabis use
While cannabis has some therapeutic benefits, those benefits must be weighed against potential harms and risks, including those related to mental health, behavioral health, pregnancy, chronic disease, and injury. While some harms/risks are linked to any cannabis use, others are tied to earlier initiation of use and more frequent use patterns. Read Dr. Schauer’s full report for the evidence on harmful effects.

Why don’t we know more about the health effects of cannabis? Cannabis is a federal schedule 1 substance in the U.S., which means it has no accepted medicinal uses and is a federally illegal substance. This has made researching cannabis and its constituents very difficult. Researchers who want to conduct studies using cannabis need a specially sanctioned lab, and have to get the cannabis they use through the federal government. These barriers have made it difficult to do the type of research that is needed to better understand the effects of cannabis and cannabinoids. 

Additionally, in studies that have been conducted, it is often difficult to tease out what is attributable to cannabis, and what is attributable to other substances – particularly tobacco and/or alcohol, both of which are also often consumed by people who use cannabis.

Finally, it’s reasonable to suspect that the amount and frequency of cannabis that someone uses, and the way in which they consume it (e.g., smoke, eat, dab, etc.) may contribute to its effects. These variables have often not been included in research (though that is starting to shift).  Most of the research reviewed above has focused on smoked cannabis, and cannabis potency that is consistent with dried flower. Little is known about the health effects of other forms of cannabis, including newer, high potency products.

Click here to read Dr. Schauer’s full report.
Go to for more information on Public Health – Seattle & King County’s priorities and efforts around cannabis prevention and education.

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