Regarding the opioids, a synergistic central nervous system sedative effect occurs in all species when opioids are combined
with sedative or anxiolytic drugs. In general, when using opioids for pain management, reduce concurrent doses of acepromazine,
alpha2 agonists, or benzodiazepines. Similarly, if the opioid is already on board, then reduce concurrent injectable and inhalant
anesthetic doses. This can be viewed as a beneficial drug interaction because opioids allow dose reductions of drugs with
cardiopulmonary depressant effects (e.g. isoflurane). No specific drug interactions with opioids preclude their use, but be mindful of additive physiologic effects
when using opioids with other medications. For example, when used repeatedly or in high doses, opioids may cause urinary retention
or gastrointestinal ileus. So in patients receiving atropine, which is known to cause ileus, and then an opioid, markedly
decreased gastrointestinal motility may occur, necessitating closer monitoring of those patients' water and food intake.
NSAIDs have more potential drug interactions that could preclude their use. The relatively selective COX-2 agents (e.g. meloxicam, carprofen) can increase the toxicity of warfarin, methotrexate sodium, valproic acid, furosemide, spironolactone,
and sulfonylureas.25 NSAIDs may decrease the efficacy of furosemide, thiazides, ACE inhibitors, and beta-blocking agents.24 Finally, relatively selective COX-2 agents are more likely to be toxic when administered with aminoglycosides, furosemide,
cyclosporine, and glucocorticoids.25
Lesley J. Smith, DVM, DACVA
Department of Surgical Sciences
School of Veterinary Medicine
University of Wisconsin
Madison, WI 53706
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