“…Other modification studies, which were mostly heroin epidemic models, took advantage of various mathematical tools to account for practical factors. For example, delayed differential equations were used to simulate processes with known durations [ 20 , 21 , 22 , 23 ], partial differential equations were utilized to incorporate the effect of age or treatment duration [ 24 , 25 , 26 , 27 , 28 , 29 ], multi-layered models were proposed when population heterogeneity was involved [ 30 , 31 , 32 , 33 ], and stochastic differential equations were formulated to reflect unexpected fluctuations in reality [ 34 , 35 , 36 , 37 , 38 ]. In addition to theoretical analyses, some synthetic drug epidemic models were applied to real settings and fitted to historical data, most of which were based on methamphetamine epidemics in South Africa [ 39 , 40 , 41 , 42 , 43 , 44 ].…”