Adolescent use: Studies show mixed findings on whether adult-use cannabis legalization affects rates of adolescent cannabis use. Demographic factors may impact this relationship such as race/ethnicity, whether parents or caregivers use/used cannabis and family income level.

Adult use: Numerous studies show that adult-use cannabis legalization is associated with an increase in cannabis use among adults, particularly among young adults (18-25). There are mixed findings if legalization is associated with an increase in cannabis use disorder diagnosis 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: No valid and reliable test is available to determine cannabis impairment among drivers)

Research studies have found mixed results about the effects of adult-use cannabis legalization on mental health issues such as psychosis, schizophrenia, depression and anxiety. Some studies have found no effects of legalization on mental health, some found that mental health problems increase with legalization, while others show decreases in some mental health issues post-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 due to edible cannabis products. There is limited evidence available 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 shown that cannabis-related emergency department (ED) visits and hospitalizations have increased after legalization of cannabis, but study results vary by type of visit, and sex/age of patients.

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. Research has identified that specific driving skills can be impaired after using cannabis. Young men and frequent cannabis users are most likely to drive after cannabis use. Use of alcohol and cannabis together clearly increases driving impairment.

Note: A valid and reliable test is not available to determine cannabis impairment among drivers

Direct causation has not been determined, however there is strong evidence that cannabis use is associated with greater risk of mental health issues (e.g., psychotic disorders, anxiety, depression) especially for consistent cannabis use in adolescence, more frequent use, and use of 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.

Studies have shown an overall moderate association between cannabis and violence. The direction of this association is not clear although some studies have found that cannabis use/intoxication can induce violent behaviors and/or alter psychological/cognitive abilities that increase the likelihood of violence in some social contexts (e.g, interpersonal violence). There are too few studies to make conclusions about the association between cannabis use and non-violent crime (e.g., property crime).

Cannabis use is associated with increased risks of stroke, heart attack and other cardiovascular problems. The risk of cardiovascular problems may be minimized with cessation of cannabis use, but more research is needed. 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 show a relationship between cannabis use and risk of specific types of cancer such as lung and testicular, particularly among heavy cannabis users, but 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.

There is mixed evidence on how cannabis use is associated with sleep. Some studies found no evidence that cannabis supports sleep, while other studies found that cannabis was associated with a decrease in insomnia. Study results vary for how cannabis affects sleep quality vs. quantity and for 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).

Numerous studies have found that cannabis use is associated with decreases in reported pain. The evidence for the degree of pain reduction is not certain, with most studies reporting small improvements in pain relief. Study findings show that cannabis is effective in reducing some kinds of pain but not others and results vary by type/amount of the cannabinoid used (tetrahydrocannabidiol [THC] which has intoxicating effects and/or cannabidiol [CBD] which is not intoxicating).

Studies have shown that cases of cannabis poisonings and cannabinoid hyperemesis syndrome (CHS) have increased in recent years with a significant proportion of these events attributed to edibles and highly concentrated cannabis products. Symptoms of cannabis poisoning can include lethargy, ataxia, decreased heart rate and agitation. Poisonings are most frequently seen in young children. CHS symptoms include nausea and repeated vomiting and are 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.

There is mixed evidence on how cannabis use affects brain development and cognition. However, it appears that the age when cannabis use begins, particularly during adolescence, and chronic or heavy use may be associated with structural changes in the brain, cognitive deficits and poor academic outcomes.