Sequential patterns of drug use initiation – can we believe in the gateway theory?

The gateway, or stepping stone, hypothesis is important as it has had considerable influence on drug policy and legislation in many countries. The gateway hypothesis offers one possible explanation for young people's development of a serious drug problem. It simply states that the use of one drug increases the risk of starting to consume another, and possibly more harmful, drug later on and that the risk increases with frequency of use (dose-response). The empirical basis for the hypothesis is the common finding that most heavy drug users have started with less dangerous drugs first and that there seems to be a "staircase" from alcohol and insolvents via cannabis and tablets to amphetamine, cocaine and heroin. The core question is whether the sequential initiation pattern of drug use is best explained by the mechanisms substantiating the gateway hypothesis or whether the phenomenon is better understood by employing the concepts of accessibility and/or transition proneness? Based on a representative sample of 21-31 year olds in Oslo we have examined the possible gateway effect of both legal (alcohol) and illegal drugs (cannabis) on subsequent use of cannabis and hard drugs (amphetamine and cocaine). We use multivariate probit models that take account of unobservable individual-specific effects to reduce the possibility of a spurious causal effect of soft drug use on the onset of hard drug use. The gateway effects were greater when we did not take account of unobserved heterogeneity, but, although substantially reduced, they remained considerable also when unobserved factors were accounted for.