"The onset of opiate antagonism following oral administration of naltrexone has been reported to be 15–30 minutes in a limited number of patients who had been receiving morphine chronically. Administration of a single 15-mg oral dose of naltrexone hydrochloride immediately following a single 30-mg subcutaneous dose of morphine has been reported to produce opiate antagonism that is prominent within 6 hours, maximal within 12 hours, and persists for at least 24 hours. The extent and duration of antagonist activity of naltrexone appear to be directly related to plasma and tissue concentrations of the drug. Plasma naltrexone concentrations of 2 ng/mL have been reported to be associated with an 87% blockade of the pharmacologic effects of a 25-mg IV dose of heroin. In one study in former opiate-dependent individuals receiving 100 mg of naltrexone hydrochloride daily and subsequently challenged with a 25-mg IV dose of heroin, the extent of blockade of the effects of heroin was 96, 87, and 47% at 24, 48, and 72 hours after naltrexone, respectively; corresponding plasma naltrexone concentrations were 2.4, 2, and 1.7 ng/mL, respectively.
The duration of the opiate antagonist activity of naltrexone appears to be dose dependent and is longer than that of equipotent doses of naloxone. A single 50-mg oral dose of naltrexone hydrochloride effectively antagonizes the pharmacologic effects of 25 mg of IV heroin or subcutaneous morphine for up to 24 hours. Increasing the dose of naltrexone hydrochloride to 100 or 150 mg reportedly antagonizes the effects of 25 mg of IV heroin for up to 48 or 72 hours, respectively. "
http://www.medscape.com/druginfo/monogr ... h&secid=10More good info:
http://www.scienceblog.com/community/ol ... 00159.html