Naltrexone is the prototypical opioid antagonist
used in absti-nence therapy, blocking the effects of heroin and other opioids
through competitive receptor inhibition. Naltrexone has no opioid agonist
effects and is a competitive opioid antagonist. It is orally effective and can
block opioid effects for 24 hours when admin-istered as a single daily dose of
50 mg; doses of 100 to 150 mg can block opioid effects for 48 to 72 hours.
Despite a favorable adverse event profile (nausea is typically the most common
side effect), naltrexone is generally not favored by opioid addicts be-cause,
unlike opioid agonists and partial agonists, it produces no positive,
reinforcing effects. Furthermore, it may be associated with the precipitation
of an opioid withdrawal syndrome if used too soon after opioid use stops, an
effect that can be minimized by administering a naloxone challenge prior to
giving the first dose of naltrexone.