Although a significant number of empirical studies provide evidence of sequencing in drug use, economic theory remains focused on addiction to a single substance. This paper presents a general model of substance use that allows for the possibility of multi-commodity habit formation and can be used to analyse the intertemporal relationship between the consumption of legal and illicit drugs, or the gateway effect. A simple two-drug model is analysed and conditions for the existence of multi-commodity habit formation are examined. It is found in the case of multi-commodity habit formation that the marginal utility of initiating a new drug is higher when there is prior consumption of the other drug. Further, it is found that the individual will initiate drug consumption with that drug that has the lowest marginal cost. The particular sequencing of drug use that is observed in empirical data is explained by differences in the marginal cost of consuming legal and illegal drugs.
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