Oral naloxone reduces constipation but not antinociception from oral morphine in the rat

Oral administration of naloxone (10 mg/kg) antagonized the slowing of the intestinal transit caused by oral morphine (1, 2.5 and 5 mg/kg) in rats. Oral administration of naloxone (10 mg/kg) did not prevent the antinociceptive effect of orally administered morphine (2.5 mg/kg) in the tail-flick test carried out on rats. It is concluded that oral naloxone locally blocks the constipating effect of morphine, while it fails to reduce the central action of morphine due to extensive metabolization after oral administration.