Background: The gateway hypothesis holds that the use of legal substances (i.e., tobacco and alcohol) increases the risk of initiating in cannabis use which, in turn, increases the chances of using other illegal substances. The validity of this hypothesis has been the subject of intense debate in recent years, finding sequences with a different order. Moreover, this pattern has been scarcely studied in Spain, where characteristics related to cannabis use are meaningfully different to other countries. This study aims to examine the gateway effects of cannabis towards other legal and illegal substances in Spanish adolescents.

Material and methods: Data were obtained from the Ministry of Health in Spain, through a representative survey of addictive behaviors of 36,984 Spanish adolescents (M_age = 15.7, SD = 1.2, 51.4% females).

Results: Lifetime cannabis use increased the likelihood of later legal substance use, both tobacco (OR = 2.0; 95%CI 1.81, 2.22) and alcohol (OR = 1.93; 95%CI 1.61, 2.31), as well as illegal substances (OR = 5.36; 95%CI 4.80, 5.98) and polysubstance (OR = 18.24; 95%CI 14.63, 22.73). Early age of cannabis use onset significantly increased the likelihood of subsequent legal and illegal substance use (ORs between 1.82 and 2.65).

Conclusions: These findings confirm and expand the available evidence on cannabis as a gateway substance. These results can help to drive preventive strategies for substance use in Spanish adolescents.