Drug Use Trajectory: The Switch from an Initial Type of Drug to a New Type of Drug

Background

Researchers and policymakers have long held the gateway hypothesis which suggests that substance use progresses from less severe drug types to more addictive types (Kandel & Kandel, 2014, 2015), and marijuana is often viewed as a gateway drug (Otten, Mun, & Dishion, 2017). Although the strong association between marijuana use and other drug use is widely documented, very limited studies have examined the use trajectory from marijuana to other drugs, and less have examined how the trajectory may vary by age groups. Furthermore, little research has considered the trajectory in which drug users start from using a drug other than marijuana. The current study is aimed at responding to the knowledge gaps.  

Methods

The study used pooled data from the 2009-2014 National Survey of Drug Use and Health, which annually surveys about 50,000 individuals 12 years and older in the U.S. civilian population. The study is limited to respondents who reported having used at least one type of drug (156,372). The variable Initial Drug Type indicates the first type of drugs used by a user (marijuana, prescriptions, inhalations, or hard drugs [e.g., heroin & opioid]). Additionally, the variable New Drug Use indicates the subsequent use of another type of drug, and the variable Interval measures the time from the use of the first drug to a new drug. Other variables include age, gender, race, education and income. Survival Analysis techniques Life table and Cox regression in SAS software are used and the estimates are weighted for national representativeness.

Results

Among the drug users, 83% started from marijuana. The remaining users started from prescription drugs (10%), inhalation (4%), or highly addictive hard drugs (3%). Life table analysis shows that 3 years after the initial use of marijuana, 60% of the users started another drug, compared to those who started from hard drugs (88%), prescriptions (72%), and inhalations (63%). Further analysis shows that youth users who were 12 to 17 were much faster than older users to switch to another type of drug. For example, 3 years after the initial use of marijuana, 83% of the youth used another type of drug, compared to 66% and 54% of those who were 18-25 and 26 or above respectively. The Cox regression models that control covariates further confirm that youth drug users were about 2 times faster to move to the use of another type of drug than users 26 years or older (p<.001).

Conclusion and implications

The majority of drug users including marijuana users will use another type of drug in 3 years after they initiated the use of a drug, and the likelihood for youth 12-17 is especially high. The findings corroborate the gateway hypothesis that drug users tend to use other drugs and marijuana users progress to use more sever drugs, which has important implications for marijuana legalization movement. The findings that youth marijuana users are quickly moving to use another type of drug is especially alarming and call for timely and effective interventions to intervene in such a trajectory.