Naltrexone (EN-1639A) is approximately 17 times more potent than nalorphine as an antagonist in man. It is virtually devoid of agonistic activity, including the ability to induce nalorphine-like dysphoric effects. Its duration of action is longer than that of naloxone, but shorter than that of cyclazocine. It is effective orally. When administered in a dose level of 50 mg/day, it produces a degree of blockade of the effects of morphine and heroin that is comparable to that obtained with 4 mg of cyclazocine per day orally. Naltrexone, thus, appears to be a relatively pure potent narcotic antagonist which is effective orally and which may have utility in the treatment of heroin and narcotic dependence.
Martin WR, Jasinski DR, Mansky PA. Naltrexone, an Antagonist for the Treatment of Heroin DependenceEffects in Man. Arch Gen Psychiatry. 1973;28(6):784-791. doi:10.1001/archpsyc.1973.01750360022003