"Originally approved for use in the treatment of opioid dependence by the United States Food and Drug administration (FDA) in 1984, naltrexone is a competitive ?-opioid receptor antagonist with negligible agonist effects, blocking euphoric and physiological effects of opioid agonists.11,12 Naltrexone does not cause the development of dependence or tolerance over time, and dosing cessation does not result in withdrawal.13

"Orally dosed naltrexone is subject to first pass metabolism, where it is converted to active (6-? naltrexol) and inactive metabolites.14 ­First-pass metabolism of orally dosed naltrexone is high, evidenced by the peak dose of naltrexone and its ­metabolites 1 hour after oral dosing.15 Serum ­half-life for chronic oral administration is approximately 10 hours.15 The half-life, when compared to naloxone, another ?-opioid antagonist, is longer, and naltrexone is able to block the agonist effects of other opioids for 48 hours.16 Oral dosing is accomplished by either 50 mg daily dosing or three times weekly dosing with two 100 mg doses and one 150 mg dose."


Kjome, Kimberly L. and Moeller, F. Gerard, "Long-Acting Injectable Naltrexone for the Management of Patients with Opioid Dependence," Substance Abuse: Research and Treatment 2011:5 1–9, doi: 10.4137/SART.S5452.