On April 26, the United States reclassified cannabis from a Schedule I controlled substance, which lacks any medical benefits, to a Schedule III controlled substance, which has accepted medical uses and lower potential for misuse than a Schedule I substance.
Over the past two decades, cannabis has been moving closer to broad legality through a series of changes at state and federal levels. Even though these changes created real benefits — including increased tax revenues and decreased cannabis-related incarcerations — people should know that regular or heavy consumption of cannabis can still pose risks, according to Emily Ansell, professor of biobehavioral health at Penn State.
For 15 years, Ansell, a Penn State Social Science Research Institute co-funded faculty member, has studied the use and co-use of three common recreational substances — cannabis, nicotine and alcohol. She led a study, published in Addictive Behaviors, examining how different cannabis products — including vapes, bongs, pipes and edibles — are associated with patterns of use that may escalate to heavier or higher risk consumption.
In this Q&A, Ansell explained the health risks and impacts associated with cannabis use and discussed how federal reclassification might help science understand cannabis better.
Q: According to research, how does cannabis use affect human health?
Ansell: Cannabis is not risk-free, and the science on that is consistent. Repeated cannabis use can lead to cannabis use disorder — a clinically recognized condition characterized by difficulty controlling one’s use despite experiencing negative consequences. Other researchers have estimated that roughly 20% of individuals who use cannabis will develop cannabis use disorder, though that prevalence may be as high as 33% for those who use regularly.
Heavy or chronic cannabis use has also been associated with cardiovascular and pulmonary health effects, impaired memory and cognition, and problems in relationships, at work and at school. A recent study found that cannabis use, whether recreational or medicinal, roughly doubled the risk of cardiovascular health problems. Repeated use can also result in withdrawal symptoms, such as irritability, sleep disruptions or cravings. This may be surprising to some as it is widely assumed that cannabis has no physical dependence potential.
Another area of concern is high-potency products such as concentrates, vapes, hash oil and some edibles. Cannabis products that exceed 70% Tetrahydrocannabinol (THC) concentration are associated with higher rates of problematic use including difficulty reducing consumption, poorer mental performance and increased likelihood of engaging in risky behaviors like driving while intoxicated.
For adolescents who use cannabis, the risk is different in kind, not just degree. The developing brain is particularly vulnerable to cannabis exposure. Regular use of cannabis during adolescence is associated with measurable disruptions to the normal development of impulse control, stress regulation and executive functioning — the ability to plan, focus, organize and regulate one’s emotions.
Q: How does cannabis compare to tobacco and alcohol in terms of health impacts?
Ansell: All three substances — cannabis, nicotine and alcohol — act on the reward processing, stress regulation and impulse control systems in the brain. Chronic, regular or heavy use of all three have known physical and mental health effects. The specific risk profiles differ, but the underlying mechanisms share more in common than most people may assume.
A common claim about cannabis that is worth addressing is the “gateway drug” framing. The evidence does not support the idea that there is something specific to cannabis that causes progression to other substances. What research does show is that any substance that alters reward and stress-regulation systems — including cannabis, alcohol or nicotine — may lower the threshold for using other or multiple substances.
Anyone who uses a recreational substance — whether cannabis, nicotine, alcohol or another drug — undergoes changes to their reward and stress regulation systems. In that limited sense, individuals who use cannabis may be more likely to use other recreational substances, but there is nothing researchers have detected about cannabis that makes individuals who use it any more susceptible to escalation of other substance use. This argument is best understood as a risk characteristic of substance use broadly rather than a property of cannabis use specifically.
Q: What happens when people use cannabis and other substances simultaneously?
Ansell: Co-use of cannabis and alcohol — particularly when consumed within about 90 minutes of one another — produces greater intoxication than either substance alone. This effect is not simply additive; it’s synergistic.
My research has shown that the more intoxicated you feel from alcohol and cannabis, one year later you are more likely you are to report an increase in hazardous cannabis use, which is use that disrupts relationships or work or use that exceeds what you intended.
Q: Do chemicals used in cannabis cultivation or harvesting represent a health concern?
Ansell: Cannabis contains over 300 distinct compounds, several of which are psychoactive. It is important to keep in mind that legal availability of any given cannabis compound does not mean that safety has been established.
Delta-8 THC is a good example of this. Delta-8 THC is one of the naturally occurring compounds in the cannabis plant that has identified psychoactive properties but is not currently regulated under federal law. Delta-8 THC occurs naturally at relatively low concentrations in cannabis, so any delta-8 products for sale have been manufactured from hemp-derived CBD. The process uses chemicals to synthesize or convert hemp derived cannabinoids to delta-8 THC, and the safety of this process or resulting products has not been established.
Potentially unsafe by-products, carcinogens or harmful contaminants resulting from the synthesis may be present in the product when it is consumed. There is no regulation of the dose in these products, and many times the labels are entirely inaccurate and may even state that the product is not psychoactive.
For all these reasons, it is not surprising that there has been an increase in adverse events reported to the Food and Drug Administration (FDA) and poison control centers related to delta-8 THC products. Consumers should really use caution when considering whether to purchase these products.
Additionally, the dangers associated with pesticide residues and heavy metals from cultivated cannabis plant are poorly understood. While this issue affects alcohol and tobacco as well to varying degrees, there is a perception that cannabis is “all natural,” a belief that is often at odds with the cultivated product or the chemical synthesis process. What remains unclear is the magnitude of consumer exposure and whether this is associated with significant health risk beyond the exposure we experience in everyday life. More systematic study is needed to establish that.
Q: How will the federal re-classification of cannabis affect research?
Ansell: For more than 40 years, federally funded research on cannabis has been conducted under tight constraints, using the same strain of the plant grown by the National Institute on Drug Abuse (NIDA). While this produced internally consistent findings and allowed comparison between studies, it remains disconnected from the cannabis products that people consume for recreational or medicinal purposes. The existing lab-based cannabis administration research does not reflect real-world potencies or the full range of available cannabis products that people actually consume.
The reclassification of cannabis from a Schedule I to a Schedule III substance is a meaningful shift for the research community. As a Schedule I substance, cannabis research required registration with the Drug Enforcement Administration, access to NIDA’s single source supply of cannabis and extensive institutional review processes that added substantial time and effort to study implementation. Those requirements made it difficult to study cannabis in ways that reflected how people actually use it.
To address this in my own research, I have used smart-phone based assessments to gather as much information about real-world cannabis use as possible. The hope is that the reclassification will allow us to bring real-world cannabis products into the research lab.
Schedule III status should theoretically reduce some of those regulatory barriers. In principle, researchers should have greater flexibility to access a wider range of cannabis products or potencies — including from licensed dispensaries — for use in controlled studies.
That said, we are still required to ensure the safety of the product being administered by meeting specific FDA standards. Like the concerns around delta-8 THC, not all products on the market are certified or approved by the FDA for consistent dose and safety. This concern, coupled with the remaining disconnect between federal regulation and state regulations, means that researchers will still need clarity on regulatory guidance, permissible product sources and how to handle interstate research collaborations given the patchwork of state-level laws. So, while this is an initial step in expanding cannabis research, there are many issues around the implementation that still need to be worked out.