ISSN 2398-2942      

Analgesia: opioid



  • Very effective analgesic agents which are safe, having minimal cardiovascular effects, if an appropriate dose is used.
  • Action produced by mimicking endogenous opiates at receptors in the central nervous system and periphery which act as transmitters in nociceptive inhibitory pathways.
  • Cause significant respiratory depression in man, this is insignificant in other animals at clinical dose rates, except when combined with other respiratory depressive agents.
  • Cause some sedation, synergistic effect with sedatives such as acepromazine Acepromazine maleate to produce profound sedation - neuroleptanalgesia.
  • Use lower doses in very old, very young dogs and those with liver disease as hepatic clearance is slowed.
  • Use with care in dogs with CNS trauma as any respiratory depression will cause an increase in intracranial pressure.
  • Many are Schedule 2 drugs being subject to strict regulations concerning purchase, storage and use.
  • Tolerance and dependence develop with prolonged administration.

Pure opioid agonists

  • Predictable analgesic effect directly related to dose.
  • High maximum efficacy.
  • If the stated dose is given and not found to be effective then the dose may be repeated safely until analgesia is attained.

Partial agonists

  • These drugs can antagonize the pure agonists listed above. They also have sufficient agonist activity in their own right to be useful analgesics.
  • Pure agonists may have a reduced effect if given following a partial agonist because the receptors will already be occupied.
  • Most are Schedule 3 drugs and so do not have such stringent regulations controlling their use.
  • There is a bell-shaped response curve for some partial agonists so that there is a ceiling to the degree of analgesia that can be produced. It is likely that this limit to the degree of analgesia is reached following administration of much higher doses than are currently clinically recommended.
  • There is also a limit to the degree of respiratory depression produced by partial agonists.
  • The analgesic effect is much less predictable than with pure agonists, and some animals may not respond.
  • Gastrointestinal side-effects such as vomiting, defecation and constipation are unlikely to occur.

Analgesic drugs - pure agonists

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Analgesic drugs - partial agonists

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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Maiante A A, Teixeira Neto F J, Beier S L et al (2009) Comparison of the cardiorespiratory effects of methadone and morphine in conscious dogs. J Vet Pharm Ther 32 (4), 317-328 PubMed.
  • Abbo L A, Ko J C, Maxwell L K et al (2008) Pharmacokinetics of buprenorphine following intravenous and oral transmucosal administration in dogs. Vet Ther (2), 83-93 PubMed.
  • Shih A C, Robertson S, Isaza N et al (2008) Comparison between analgesic effects of buprenorphine, carprofen, and buprenorphine with carprofen for canine ovariohysterectomy. Vet Anaesth Analg 35 (1), 69-79 PubMed.
  • Capner C A, Lascelles B D, Waterman-Pearson A E (1999) Current British veterinary attitudes to perioperative analgesia for dogs.​ Vet Rec 145 (4), 95-99 PubMed.
  • Johnson C (1999) Chemical restraint in the dog and cat. In Practice 21 (3), 111-118 VetMedResource.
  • Pibarot P, Dupuis J, Grisneaux E et al (1997) Comparison of ketoprofen, oxymorphone hydrochloride, and butorphanol in the treatment of post operative pain in dogs. JAVMA 211 (4), 438-444 PubMed.
  • Sammarco J L, Conzemius M G, Perkowski S Z et al (1996) Post-operative analgesia for stifle surgery - a comparison of intra-articular bupivacaine, morphine, or saline. Vet Surg 25 (1), 59-69 PubMed.
  • Schultheiss P J, Morse B C, Baker W H (1995) Evaluation of a transdermal fentanyl system in the dog. Contemp Top Lab Anim Sci​ 34 (5), 75-81 PubMed.

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