Opioids are the most effective drug class for managing acute pain and can play a role in managing chronic pain. An improved understanding of neuropharmacology and the development of novel formulations of opioids makes it incumbent on veterinarians to remain familiar with their modes of action; various subtypes within this drug class; and the prevention, recognition, and treatment of adverse effects. While a complete discussion of opioids is beyond the scope of these guidelines, the Task Force makes the following recommendations for using this class of drugs in dogs and cats:

• Opioids should be used as a routine preoperative medicant, preferentially in combination with a tranquilizer/sedative (e.g., acepromazine, midazolam, diazepam, or α-2 adrenergic agonist such as dexmedetomidine) when the patient’s condition warrants their use. Full μ agonists elicit greater and more predictable analgesia than partial μ agonists or κ agonists. In dogs, the μ antagonist/ κ agonist  butorphanol in particular appears to have limited somatic analgesia and very short duration of visceral
  analgesia.^12,13
• In a comparison study, buprenorphine administered before surgery and during wound closure provided adequate analgesia for 6 hr following ovariohysterectomy in cats, whereas butorphanol did not. ¹⁴
• In cats, the subcutaneous route of opioid administration is not recommended. IM and IV routes are preferred both pre- and postoperatively. ¹⁵ The oral transmucosal or buccal route of administration for buprenorphine may also have clinical efficacy as well. ^16,17
• The individual effect of any opioid, including duration, may vary widely from patient to patient. Postoperative reevaluation should be made frequently to determine ongoing opioid requirements.
• For a patient undergoing major surgery, whereby ongoing opioid administration can be anticipated, the clinician may choose from the following strategies:
  
  • Periodic readministration of parenteral opioids.
  • Constant or variable rate infusion. Calculators can be found online.
  • Long-acting formulations and technologies. For dogs there is an FDA- approved transdermal fentanyl product^a. Given this canine fentanyl product on the market, the Task Force discourages the use of human commercial fentanyl patches in dogs due to highly variable pharmacokinetics, risk of either accidental or purposeful human exposure, with potential liability for extralabel use. There is not an expert consensus regarding the utility of fentanyl patches in cats. The FDA has more recently approved a concentrated injectable buprenorphine product for cats^b, which has been formulated to provide a 24 hr duration of action when administered as directed.
  • Oral opioids. Dogs exhibit a robust first-pass effect of oral opioids. No clinical studies document efficacy, but pharmacokinetics of codeine and hydrocodone suggest possible utility. ¹⁸ No comparable studies exist for cats. Opioids are synergistic with α-2 adrenergic agonists, allowing them to be used in low-dose combinations, either with or without ketamine, to great effect for both sedation and analgesia.
  • Opioids play a significant role in human medicine for the treatment of chronic pain and may play an underappreciated role in dogs and cats as well, especially for cancer-related pain and in palliative care patients. That said, clinicians must be vigilant with regard to long-term adverse effects such as constipation, drug tolerance, and the potential for diversion by clients.

^a Recuvyra; Elanco Animal Health, Greenfield, IN

^b Simbadol; Abbott Laboratories, Abbott Park, IL