“…The current study builds upon previous findings and demonstrates that 2 hours of access to 2.5 µg/kg/infusion of fentanyl over 14 days is sufficient to establish escalation of fentanyl intake and result in withdrawal symptoms in mice. Previous literature has investigated rodent models of opioid dependence, including morphine (47)(48)(49), oxycodone (10,12,50,51), heroin (6,(52)(53)(54), remifentanil (34,(55)(56)(57)(58), and fentanyl (24,33,36,38,(59)(60)(61)(62)(63)(64)(65)(66), using various routes of administration, with the majority of studies done in rats. IVSA of fentanyl has been widely studied in rats (24,25,(59)(60)(61)(62)(67)(68)(69)(70)(71), with findings demonstrating mixed results on the development of dependence and withdrawal signs.…”