In this article, we propose a mathematical model that describes the dynamics of a population divided into susceptible drinkers, moderate drinkers, and heavy drinkers subject to an external influence. The external influence is modelled using a supplementary dynamical variable that is not a group of individuals but that enters the equations affecting the choices of the population classes.The system we define can be investigated using two simplified systems (one of which is a real subsystem), which model the populations of susceptible and moderate drinkers or susceptible and heavy drinkers independently. The dynamics of these two subsystems can be described exhaustively. The full system is too rich in possible scenarios, but its qualitative behaviour is connected to that of the two simplified systems. We make a complete description only in one particular case by means of numerical simulations. Copyright © 2017 John Wiley & Sons, Ltd.