Revisiting the Gateway Drug Hypothesis for Cannabis: A Secondary Analysis of a Nationwide Survey Among Community Users in Japan

Abstract

Aim: Cannabis has historically been used for medicinal and industrial purposes, but is strictly regulated worldwide due to the psychoactive effects of THC. In Japan, cannabis is frequently labeled a "gateway drug," yet strong causal evidence for progression to other substances is limited. This study investigates whether cannabis acts as a gateway drug among Japanese users.

Methods: An anonymous online survey was conducted in January 2021 with 3900 individuals reporting lifetime cannabis use in Japan. Participants were recruited via social media. The survey gathered data on demographics, cannabis and other substance use history, order of substance initiation, psychiatric background, and criminal records. A Sankey diagram visualized substance use progression, and odds ratios were calculated to assess the likelihood of using other substances following cannabis use.

Results: Of all respondents, 81.5% were male, with the largest age group being 20-24. Tobacco and alcohol were the most common initial substances, while cannabis was typically the third. Odds for subsequent use of alcohol, tobacco, methamphetamine, and other illicit drugs after cannabis use were 1.25, 0.77, 0.08, and 0.78, respectively, suggesting low probabilities of progression. Nearly half of those who reported cannabis as their third drug did not use other substances afterward.

Conclusion: Cannabis use in Japan typically follows alcohol and tobacco, and rarely leads to further drug use. These findings challenge the gateway hypothesis in the Japanese context. Shared vulnerabilities and strict drug policies may shape these patterns. Further research is warranted to explore the impact of legal changes on drug use behavior.

Registry and registration number of the study/trial: Not applicable.

Keywords: Japan; cannabis; gateway drug hypothesis; marijuana; substance use.

FIGURE 1

Sankey diagram illustrating the sequential patterns of substance use among community‐based cannabis users in Japan (N = 3900). The diagram displays the progression from the first to the fifth substance used, including cannabis. The width of each flow represents the number of respondents reporting a given transition. Tobacco (48.1%) and alcohol (42.3%) were the most common initial substances, followed by benzodiazepines and cannabis. Cannabis was most frequently used as the third substance (68.2%). Notably, 54.7% of individuals who used cannabis as their third substance reported no further transition to other substances. This visualization highlights that cannabis use in Japan typically follows the use of alcohol and tobacco and does not commonly lead to the use of other substances.