Agonist/Antagonist Analgesics

Pentazocine (PZ), buprenorphine, butorphanol, cyclazocine, and nalbuphine are known to have opioid agonistic and opiate antagonistic effects. The analgesic efficacies of these drugs are generally high, and the relative potencies against morphine in humans are, in the above order, 1/3, 20-40, 5, 10-20, 1, and 1/7-1/10. The relative potencies of the antagonistic effect against nalorphine in humans are 1/50, 10, none, 5-7, 1, and 1/7-1/10 respectively. However, both in animals and humans, the synergism between morphine and PZ is observable when the doses of both drugs are relatively low.