Why are nubain and phenergan (promethazine) administered togetherl?

Synergistic. Nubain is semi-synthetic opiod analgesic while Phenergan (promethazine) is an antihistamine. The Phenergan (promethazine) is used in the postoperative period to reduce nausea and to potentiate the pain killing effects of the nubain. The hope is to use less Nubain and avoid its possible side effects by using the Phenergan (promethazine) with it.