The Study System is a single-use system that consists of two parts: a Drug Unit containing 10.8 mg fentanyl hydrochloride, and a Controller that supplies the dosing current. The two parts are packaged separately and assembled immediately prior to use. The Study System will be applied to the upper, outer arm of each subject. Fentanyl will be delivered from the Study System via transdermal iontophoresis using a current of 170 mcAmp (Treatment B), 140 mcAmp (Treatment C), 200 mcAmp (Treatment D), or 230 mcAmp (Treatment E). Each subject will be randomly assigned to a treatment sequence consisting of Treatment A (IV fentanyl citrate), Study System Treatment B (170 mcAmp), and one of three additional Study System treatments consisting of Treatment C (140 mcAmp), Treatment D (200 mcAmp), or Treatment E (230 mcAmp). Each subject will therefore receive a total of three treatments (A and B and either C, D, or E).~The specific treatments are outlined in detail below:~Treatment A: Fentanyl citrate (equivalent to 80 mcg fentanyl) administered over 20 minutes by IV infusion every hour through 23.33 hours.~Treatment B: Two consecutive 40 mcg fentanyl doses each delivered over 10 minutes by the Study System (170 mcAmp) every hour through 23.33 hours.~Treatment C: Two consecutive 35 mcg fentanyl doses, each delivered over 10 minutes by the Study System (140 mcAmp) every hour through 23.33 hours.~Treatment D: Two consecutive 50 mcg fentanyl doses, each delivered over 10 minutes by the Study System (200 mcAmp) every hour through 23.33 hours.~Treatment E: Two consecutive 54 mcg fentanyl doses, each delivered over 10 minutes by the Study System (230 mcAmp) every hour through 23.33 hours.~Naltrexone (50 mg) will be administered orally (PO) every 12 hours, beginning 14 hours before the start of each fentanyl treatment and ending approximately 11 hours after completion of each treatment 
This study is a single-center, randomized, open-label, 3-period, 5-treatment, 6-sequence design. A total of 54 eligible subjects will receive three total treatments; one with intravenous (IV) infusion and two with the Study System. Each subject will be randomly assigned to receive a treatment sequence consisting of Treatment A (IV fentanyl citrate), Study System Treatment B (170 mcAmp), and one of three additional Study System treatments (140 mcAmp or 200 mcAmp or 230 mcAmp) 