Comparison of behavioral effects of morphine and fentanyl in dogs and cats.

Behavioral effects induced by intravenous administration of morphine at 0.3, 0.6, 1.2, and 2.4 µg/kg and fentanyl at 5, 10, 20, and 40 µg/kg were evaluated in dogs and cats. In dogs, fentanyl and morphine depressed activity and level of consciousness in a dose- dependant manner. In cats, higher doses of fentanyl stimulated activity temporarily, but excitement, so-called "opioid mania," was not observed. Morphine induced distinctive behavioral changes characterized by sitting with fixed staring, and "opioid mania" was not observed in cats.