Elevation in analgetic effect and plasma levels of morphine by desipramine in rats.

Pretreatment of rats with desipramine (30 mg/kg, s.c.) enhanced both peak intensity and duration of morphine sulfate (5 mg/kg, s.c.) analgesia as determined by measurement of tail flick latency. Morphine concentrations in plasma were also significantly elevated after desipramine. Increased narcotic analgetic responses following desipramine pretreatment appear related to higher circulating levels of unmetabolized morphine.