Experts agree that the use of marijuana is not causally linked to the use of other illicit substances
"There is compelling and enduring evidence that marijuana is not a gateway drug, ... Yet, non-evidence-based political factors on both the left and the right remain the reason for the persistence of the gateway myth."
Sequentially, most consumers of illicit substances first experimented with either alcohol or tobacco
"Alcohol is the most commonly used substance, and the majority of polysubstance using respondents consumed alcohol prior to tobacco or marijuana initiation. Respondents initiating alcohol use in sixth grade reported significantly greater lifetime illicit substance use and more frequent illicit substance use than those initiating alcohol use in ninth grade or later. ... Our results ... assert that the earlier one initiates alcohol use, the more likely that they will engage in future illicit substance use."
"Substance use tends to decline towards the end of young adulthood. This decline is true for alcohol as well as illicit drug use. ... For marijuana, ... peak use occurred at about age 19, remained stable for the next four to five years, and then declined."
In jurisdictions where marijuana is legally accessible, adults typically report decreasing their use of other controlled substances. In this sense, marijuana appears to act more as a potential ‘exit drug’ rather than as an alleged 'gateway'
"[F]indings on cannabis substitution effect and the biological mechanisms behind it strongly suggest that cannabis could play a role in reducing the public health impacts of prescription and non-prescription opioids. ... The growing body of research supporting the medical use of cannabis as an adjunct or substitute for opioids creates an evidence-based rationale for governments, health care providers, and academic researchers to consider the implementation and assessment of cannabis-based interventions in the opioid crisis."
"Among respondents that regularly used opioids, over three-quarters (76.7%) indicated that they reduced their use since they started medical cannabis. This was significantly ( p < 0.0001) greater than the patients that reduced their use of antidepressants (37.6%) or alcohol (42.0%). Approximately two-thirds of patients decreased their use of anti-anxiety (71.8%), migraine (66.7%), and sleep (65.2%) medications following medical cannabis."
"Findings include high self-reported use of cannabis as a substitute for prescription drugs (63%), particularly pharmaceutical opioids (30%), benzodiazepines (16%), and antidepressants (12%). Patients also reported substituting cannabis for alcohol (25%), cigarettes/tobacco (12%), and illicit drugs (3%)."
"These patient-reported outcomes support prior research that individuals are using cannabis as a substitute for prescription drugs, particularly, narcotics/opioids, and independent of whether they identify themselves as medical or non-medical users. This is especially true if they suffer from pain, anxiety and depression."
"Substituting cannabis for one or more of alcohol, illicit drugs or prescription drugs was reported by 87% of respondents, with 80.3% reporting substitution for prescription drugs, 51.7% for alcohol, and 32.6% for illicit substances. ... The finding that cannabis was substituted for all three classes of substances suggests that the medical use of cannabis may play a harm reduction role in the context of use of these substances, and may have implications for abstinence-based substance use treatment approaches."
In clinical settings, marijuana use is associated with reduced cravings for cocaine and opiates
"In this longitudinal study, we observed that a period of self-reported intentional use of cannabis ... was associated with subsequent periods of reduced use of crack [cocaine]. ... Given the substantial global burden of morbidity and mortality attributable to crack cocaine use disorders alongside a lack of effective pharmacotherapies, we echo calls for rigorous experimental research on cannabinoids as a potential treatment for crack cocaine use disorders."
"Most of the subjects in our study ceased to use crack cocaine and reported that the use of cannabis had reduced their craving and produced subjective and concrete changes in their behavior helping them overcome addiction."