Adolescent use: Although research shows a mix of findings for the effects of adult-use cannabis legalization on adolescent cannabis use, most studies show no increase in use among adolescents following legalization. Demographic factors may impact this relationship, such as family socioeconomic status and whether parents or caregivers use cannabis.

Adult use: Numerous studies show that adult-use cannabis legalization is associated with an increase in cannabis use among adults, particularly young adults, however, this was not always consistent across demographic characteristics such as sex, race/ethnicity and income. There are mixed findings if legalization is associated with an increase in cannabis use disorder and treatment.

Studies show mixed findings on whether cannabis legalization affects rates of traffic crashes. Some studies found legalization of adult-use cannabis was associated with an increase in traffic crashes, a few studies found no change or a decrease in crashes, while others studies reported varied results depending on factors such as type of crash (such as fatal vs. non-fatal) and age/sex of driver. Driving after cannabis use has generally been found to be more common post-legalization of adult-use cannabis.

(Note: Currently, there is no valid and reliable test available to determine cannabis impairment among drivers

Research findings on the relationship between adult-use cannabis legalization and mental health are mixed. Some studies found a higher prevelence of certain conditions following legalization, such as suicide deaths, schizophrenia, and depression. Other studies found no increase in these conditions, or even reported improvements in certain mental health conditions in certain populations following legalization.

No clear conclusions can be drawn regarding whether adult-use cannabis legalization impacts crime, with some studies showing increases, some showing decreases, and others showing no change in crime after legalization. Results of these studies also vary by the type of crime. There is some evidence that adult-use cannabis legalization may allow law enforcement to focus on other crimes (e.g., improving clearance rates for other criminal activity) and legalization may lead to a decrease in demand for illegal drug markets.

Numerous studies show that rates of unintentional cannabis poisonings have increased following legalization of adult-use cannabis. These increases are more prominent among young children, and are primarily the result of children accidentally ingesting edible cannabis products, which often mimic the appearance and taste of candy. There are too few studies available to make a conclusion on whether legalization has affected rates of cannabinoid hyperemesis syndrome (CHS), which is characterized by repeated vomiting and nausea due to chronic heavy cannabis use.

Several studies have found that cannabis-related Emergency Department (ED) visits and hospitalizations increased after legalization of adult-use cannabis. The rate of increase was found to vary by demographic factors (such as age group and sex), location, and chief complaint at time of presentation or admission.

Studies showed mixed results for the relationship between adult-use cannabis legalization and opioid-related outcomes (opioid prescriptions or opioid-related mortality/outcomes). There is some evidence that medical cannabis legalization is associated with decreased opioid prescriptions. No clear relationship has been established between adult-use cannabis legalization and alcohol use with results varying by factors such as age and sex.

Although some studies have found that cannabis use is associated with an increased risk of traffic crashes, several studies demonstrate no risk or mixed results. Results from studies examining the impact of cannabis use on driving performance are also mixed, however it appears that cannabis use impairs some driving skills. The duration of this impairment after cannabis use is unknown but some studies suggest the impairment may last over 5 hours. Young men and frequent cannabis users are most likely to drive after cannabis use, and use of alcohol and cannabis together further impairs driving ability. Studying the impact of cannabis on driving performance is difficult for a number of reasons, including the inability of driving simulations to capture real-world conditions and lack of a reliable test to determine cannabis impairment.

Direct causation between use of cannabis and mental health issues has not been determined, however there is strong evidence that cannabis use is associated with greater risk of some mental health issues (e.g., psychotic disorders, anxiety, depression). This risk appears higher for individuals who consistently use cannabis in adolescence, use cannabis very frequently, and use high potency THC-based cannabis (THC = tetrahydrocannabidiol, the component in cannabis that causes intoxicating effects). There are potential factors that affect the relationship between cannabis use and poor mental health, including pre-existing vulnerability to psychotic disorders, age when cannabis was first used, and using cannabis with other substances. Some studies report that cannabis use may improve certain mental health symptoms (e.g., anxiety, post-traumatic stress disorder) but more research is needed.

There are a limited number of studies on association between cannabis use and violence/crime. An overall moderate association has been found between cannabis use and violent behavior; however, results have been mixed and the strength and direction of the associations are not fully understood. Some studies have found that cannabis use and intoxication can potentially alter cognitive abilities in some individuals (e.g. causing paranoia and psychosis) which can in turn increase the likelihood of violence in some social contexts. There are too few studies to make conclusions about the association between cannabis use and non-violent crime (e.g., property crime).

Studies have found that cannabis use is associated with increased risks of stroke, heart attack and multiple other cardiovascular diseases. The risk of developing cardiovascular problems may be minimized with cessation of cannabis use, but more research is needed to determine the cardiovascular effects of long-term cannabis use. Cannabis appears to pose a unique risk for cardiovascular events in otherwise young, healthy people.

There is not enough evidence to determine the extent of the relationship between cannabis use and development of cancer. Some studies found cannabis use increased the risk of developing specific types of cancers such as lung, testicular, and throat and mouth cancers, particularly among heavy cannabis users. However, this may be complicated by factors such as co-use of tobacco and challenges in accurately measuring cannabis use. Among those with existing or past cancer diagnosis, cannabis may be used as a way to cope with nausea/vomiting, appetite and pain, particularly while receiving cancer treatment.

Findings from studies examining the effects of cannabis use on sleep have been mixed. Some studies found that cannabis use reduced sleep quality, while other studies found that cannabis use improved sleep for individuals with insomnia. Study results vary on how cannabis affects both the quality of sleep and length of sleep. Variation was also found between the type/amount of cannabinoid used (tetrahydrocannabidiol [THC] which has intoxicating effects and/or cannabidiol [CBD] which is not intoxicating). Using cannabis as a sleep aid may result in a cycle of increased use and a high tolerance to cannabis (i.e., needing more cannabis to create the same effect).

Results from studies examining the relationship between cannabis use and pain have been mixed. Numerous studies have found that cannabis use is associated with a modest decrease in reported pain levels, however evidence for the degree of pain reduction is uncertain. Other studies found that cannabis use was associated with an increase in reported pain. Findings suggest that cannabis may be effective in reducing some types of pain but not others, and that the type of cannabis/amount of CBD and THC may also play a role.

Studies have shown that cases of cannabis poisonings and cannabinoid hyperemesis syndrome (CHS) have increased in recent years. A significant amount of this increase has been attributed to edibles and highly concentrated cannabis products. The increase is especially notable in young children, who can accidentally ingest edible cannabis products that are designed to look and taste like candies. CHS is more common among females, adolescents and young adults. Overall, there is a need for increased education of clinical symptoms and treatment/management of cannabis poisonings and CHS.

Research findings on the effects of cannabis use on brain development and cognition are mixed, however most evidence suggests cannabis may have a variety of adverse effects on brain development and cognition. Effects include slower reaction time, poor memory, poor academic achievement, social difficulties, and potential structural change in the brain. These effects appear more acute in adolescents and those with pre-existing neurological disorders. It also appears that the age when someone began using cannabis, using particularly strong types of cannabis, and heavy use may be associated with more significant structural changes in the brain and cognitive deficits.