CVC Highlight: Busting two common opioid myths

Myth #1: Morphine cannot be used in cats because of central nervous system excitement.

Morphine mania was the name given to the dysphoric manic state that cats can experience when given high doses of morphine. Because of this, the myth arose that cats can't tolerate narcotics. However, morphine mania was documented from doses that are not used clinically.

Butch KuKanich, DVM, PhD, DACVCP

Morphine is often used in cats without severe adverse effects or central nervous system excitement; the effects are dose-dependent. Morphine mania can be produced in cats by administering 5 to 20 mg/kg morphine subcutaneously; this dose is 20 to 100 times the clinically recommended dose (0.1 to 0.25 mg/kg given IV, IM, or SC every two to four hours). High doses of opioids in any species can produce central nervous system excitement and seizures.

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Morphine metabolism is slightly slower in cats than in dogs (the plasma half-life in cats is about 1.3 hours vs. 1 hour in dogs). But the key reason cats require a decreased dosage is that their volume of distribution is smaller than in dogs. The lower volume of distribution results in higher plasma concentrations being achieved with a given dose. Since the half-life is similar in the two species, the dosing interval is the same.

Myth #2: The most common adverse effects of opioid administration are cardiovascular and respiratory depression.

Contrary to popular belief, the most common adverse effects of opioids don't involve cardiovascular or respiratory depression. The most common adverse effects include hypothermia, sedation, nausea, vomiting, panting, and defecation.

Although opioids can cause pronounced bradycardia, cardiac output remains stable because of increased stroke volume in animals administered clinically recommended dosages. Remember that cardiac output is the product of stroke volume and heart rate, so there is no net change in blood pressure or tissue perfusion in these cases.

A dose-dependent respiratory depression does occur with opioid administration, but with clinically recommended dosages, the respiratory depressant effects are clinically insignificant. Keep in mind that mild respiratory depression can be a concern in animals with head trauma or pre-existing pulmonary diseases, or in combination with other respiratory depressants.

Butch KuKanich, DVM, PhD, DACVCP

Department of Anatomy and Physiology

College of Veterinary Medicine

Kansas State University

Manhattan, KS 66506