The current study presents the most recent changes in marijuana use in the USA during a period of particularly high legalisation. It finds that while lifetime marijuana use, and first use before the age of 18, has remained stable, the overall prevalence of past-year marijuana use has significantly increased over the 14-year period. While past-year use was still more common among younger age groups, males, non-Hispanic blacks and those with lower income, significant trends of increasing past-year use were seen in all age categories, males and females, all racial categories, highly educated populations and all income levels. Age-specific marijuana use trend in response to legalisation laws has been studied elsewhere; but with inconsistent findings. There has been growing consensus that increasing recent-use of marijuana is seen among late to middle-aged adults after legalisation6 13; a recent study by Salas-Wright and colleagues showing trends of increasing past-year use among late to middle-aged adults in the USA between 2002 and 2014,5 findings that are supported by the present study. Whether this pattern of recent use among older populations is associated with increasing marijuana-use for medicinal purposes (seen most prominently among older, white, male and high-income populations in the USA),14 is unclear, and such data were not available for analysis in the present study. However, there has been less consensus about recent-use trends among adolescent populations. While a comprehensive study by Harpin et al described no change in adolescent-use after legalisation10 (further supported by more recent findings6), a 2019 review by Bae and Kerr described increasing recent-use among adolescents in the USA after legalisation.8 Contrastingly, a 2016 US study by Grucza et al describes that past-year marijuana use actually decreased among adolescents, during the highest periods of legalisation.12 Information about marijuana-use trends in adolescents less than 18 years old was not publicly available on NHANES for the present study, but it has been noted that over the 14-year study period, there has been no significant change in reported first use before the age of 18 (table 1).

Our findings of younger, male, non-Hispanic black and lower-income populations being most likely to use marijuana overall, are also consistent with previous findings not aforementioned.4 7 Native American populations, those living in urban areas, and those living in western states, have also been shown to be more likely to have recently used marijuana; but this information was not available on NHANES in sufficient detail, for the present analyses.

In addition to demographic risk factors indicating potential marijuana use, an understanding of other risk factors is important for those involved in fields of public policy and healthcare. While age and gender remain somewhat inconsistently described risk factors, tobacco smoking is a demonstrated risk factor for marijuana use. Though not analysed in the present study, a recent study demonstrated that current smokers have almost 6 x increased odds of recent marijuana use compared with non-smokers,18 outlining another subpopulation particularly at risk. Another group at risk of increasing marijuana use are non-medical users of prescription drugs (NMUPD). A recent study by Karjalainen et al demonstrated significantly increasing trends of illicit drug use among NMUPD (92% of whom had used cannabis in the last year), that could not otherwise be explained by age or gender.19

Important to consider in the context of the present findings is that marijuana-use trends may not to be restricted only to states with marijuana legalisation. In 2017, Hasin et al demonstrated significantly increased marijuana use both in states with and without marijuana legalisation laws,4 7 although fractionally higher in states where marijuana was legalised. However, a recent analysis by Han et al demonstrated a significantly higher trend of increasing medical-marijuana use in states without legalisation (adjusted OR, AOR 1.4, 95% CI 1.05 to 1.90), compared with states with legalisation (AOR 1.3, 95% CI 1.03 to 1.61);14 underlining the fact that evolving legalisation laws are also of relevance to states where recreational and medical marijuana use remains illegal.

Exploring the economic and societal cost-effectiveness of marijuana legalisation is also important for those involved in healthcare policy and decision making. Marijuana legalisation was posited to lower price, increase availability and thereby increase marijuana use,6 with early fears that profit motive would take precedence over public health issues.20 21 The retail price of marijuana on the legal cannabis market had sharply fallen by almost 70%, just 3 years after legalisation in Washington state.22 23 Some reports described an initial increase in self-reported street prices of marijuana in response to legalisation as demand increased, by up to 36%,24 with limited price change thereafter. However, similarly to results aforementioned, medical legalisation appears to have affected only adult marijuana use, with minimal significant changes to adolescent use.13 While studies of the effect of recreational marijuana legalisation on its use are still emerging, there appears to be no/minimal effect on adolescent or college marijuana use.13 The passage of legalisation laws also offers an important social justice benefit25; by removing mechanisms for unfair, damaging disparities in law enforcement.26 While more permissive marijuana laws may appeal to social justice aims (reducing racial disparity in law enforcement), and increase revenue to state and local government through taxation,6 the public health trade-offs and overall costs of use-related adverse physical and psychosocial consequences27 in response to changing laws remains difficult to accurately describe.6