Naltrexone

"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."