We　study　a　switching　heroin　epidemic　model　in　this　paper,　in　which　the　switching　of　supply　of　heroin　occurs　due　to　the　flowering　period　and　fruiting　period　of　opium　poppy　plants.　Precisely,　we　give　three　equations　to　represent　the　dynamics　of　the　susceptible,　the　dynamics　of　the　untreated　drug　addicts　and　the　dynamics　of　the　drug　addicts　under　treatment,　respectively,　within　a　local　population,　and　the　coefficients　of　each　equation　are　functions　of　Markov　chains　taking　values　in　a　finite　state　space.　The　first　concern　is　to　prove　the　existence　and　uniqueness　of　a　global　positive　solution　to　the　switching　model.　Then,　the　survival　dynamics　including　the　extinction　and　persistence　of　the　untreated　drug　addicts　under　some　moderate　conditions　are　derived.　The　corresponding　numerical　simulations　reveal　that　the　densities　of　sample　paths　depend　on　regime　switching,　and　larger　intensities　of　the　white　noises　yield　earlier　times　for　extinction　of　the　untreated　drug　addicts.　Especially,　when　the　switching　model　degenerates　to　the　constant　model,　we　show　the　existence　of　the　positive　equilibrium　point　under　moderate　conditions,　and　we　give　the　expression　of　the　probability　density　function　around　the　positive　equilibrium　point.