For major surgeries, my opioid preference in cats is oxymorphone (0.05 to 0.1 mg/kg intramuscularly or intravenously). For animals that are medically compromised, geriatric, or just mellow, I find oxymorphone provides moderate sedation in combination with low doses of acepromazine maleate (i.e. 0.01 to 0.05 mg/kg) or midazolam hydrochloride (0.1 mg/kg) given intramuscularly for preanesthetic medication and preemptive analgesia. Acepromazine should be used at the lowest suggested dose (i.e. 0.01 mg/kg) if the cat has severe systemic disease. During surgical recovery after procedures I presumed to be painful, I then repeat a half to a full dose of the initial oxymorphone dose to maintain good analgesia.

The concern about side effects with morphine stems largely from literature dating from the late 1800s and early 1900s when 100-fold doses of the drug were given experimentally to animals. These doses caused severe cardiorespiratory depression and bizarre behavioral effects. It is true that the agonist opioids can cause excitement or euphoria in some cats. This effect seems to be an individual cat response. It is more often seen in young, healthy cats and is more obvious with higher doses. In my experience, this side effect is usually moderated by adding a sedative, such as acepromazine, or an anxiolytic, such as midazolam. If the cat has undergone a painful procedure, I am reluctant to reverse the agonist opioid with a drug such as butorphanol or naloxone hydrochloride. The cat may seem better because the butorphanol or naloxone relieves the cat's dysphoria. The cat, however, may in fact be more uncomfortable and may exhibit subtle signs of pain that are easy to ignore now that it is quiet. Interestingly, research on butorphanol's behavioral effects in healthy cats not experiencing pain has shown that butorphanol can cause more dysphoria than the pure agonist opioid hydromorphone.^5,6

A common misconception, again based on old literature, is that morphine and other agonist opioids cause respiratory depression. To my knowledge, no recent studies have examined the incidence of respiratory depression in cats receiving clinically relevant doses of opioids; the clinical impression is that there are not significant increases in partial pressure of carbon dioxide (P_aCO₂) or decreases in partial pressure of oxygen (P_aO₂) in arterial blood after administration of opioids within the reference ranges suggested.

Both lidocaine and bupivacaine have a relatively narrow margin of safety in cats.^7,8 As antiarrhythmic drugs, they are rarely, if ever, indicated for use in cats. Both are useful for local anesthetic infiltration, particularly as a ring block for declaw procedures. Lidocaine has caused signs of toxicosis in cats (e.g. central nervous system depression, seizures, cardiovascular depression) at intravenous doses as low as 0.5 mg/kg.⁷ Doses of lidocaine approaching 10 mg/kg when infiltrated subcutaneously for local anesthetic blocks may cause neurologic signs in cats.⁸ In my clinical experience, bupivacaine doses less than 2 mg/kg, infiltrated subcutaneously, are generally safe. To be safe, always calculate the total dose of the drug in mg/kg before administering local anesthesia in cats. If it is necessary to administer a certain volume, dilute the drug with 0.9% saline solution to keep the volume adequate while limiting the mg/kg dose.