The article "Threshold Analysis of a Drug Use Epidemic Model" presents a mathematical model that describes the dynamics of a drug use epidemic in a population. The authors formulate a model of the individual's response to the drug based on the interaction between dosage and the active and inactive zones in the individual's body. They then consider a population of individuals, classified into non-users (susceptible), active users (infected), and individuals who abandon drug use (removed) through treatment, cure or disenchantment, etc. We assume that recruitment of susceptibles to active drug use occurs through contact with active users, and the effectiveness of recruitment (or infectiousness) depends on the user's age as a user. Thus, the model is based solely on the user's response to the drug, and shows us that when some combination of some parameters (susceptible population size and individual reaction) do not exceed a critical threshold value, there will be few users; but when the threshold value is exceeded, an epidemic of drug use occurs.