Introduction

• Almost throughout all of recorded history the opium poppy (Papaver somniferum) has been used to provide pain relief in one form or another. Despite the development of a wide range of naturally occurring and synthetic opioids, in many cases, morphine continues to be the mainstay of analgesic therapy for the treatment of severe pain in both human and veterinary medicine.
• The use of opioids in horses is a matter of some discussion and occasional controversy. Continued research into the role of opioids in the treatment of equine pain is needed.

Opiate

• Drugs derived from the opium poppy, eg morphine, papaveretum and codeine.

Narcotic

• Any drug that produces insensibility or stupor.

Opioids

• Strictly defined, an opioid is a synthetic narcotic that has opiate-like activity but is not derived from opium. Opioid is frequently used to describe any drug with opiate-like activity.
• Narcotic analgesics are opioids that provide analgesia but also induce a state of sedation or euphoria (a sense of well-being) called narcosis.
• Opioids exert their effects by binding to opioid receptors.
• Originally it was thought that opioid receptors could only be found in the brain and the spinal cord. Subsequent research has identified opioid receptors in the periphery, eg gastrointestinal tract, cornea, dental tissue and joints. The population of peripheral opioid receptors is fluid and increases in conditions of inflammation.
• Opioid receptor distribution in the CNS and the effects of these receptors differs between species (and probably individuals). In humans µ agonists tend to cause narcosis whereas in horses they may be associated with an increase in locomotor activity and excitement.

Receptor types

Side effects

Classification of opioids

Clinical use of opioids

Metabolism

Further Reading

Publications

Refereed papers

• Recent references from PubMed and VetMedResource.
• Tessier C, Pitaud J P, Thorin C & Touzot-Jourde G (2019) Systemic morphine administration causes gastric distention and hyperphagia in healthy horses. Equine Vet J 51 (5), 653-657 PubMed.
• Gozalo-Marcilla M, Lune S P L & Crosignani N (2017) Sedative and antinociceptive effects of different combinations of detomidine and methadone in standing horses. Vet Anaesth Analg 44 (5), 1116-1127 PubMed.
• Knych H K (2017) Nonsteroidal Anti-inflammatory Drug Use in Horses. Vet Clin North Am Equine Pract 33, 1-15.
• Talyor P M, Hoare H R, de Vries A, Love E J, Coumbe K M, White K L & Murrell J C (2016) A multicentre, prospective, randomised, blinded clinical trial to compare some perioperative effects of buprenorphine or butorphanol premedication before equine elective general anaesthesia and surgery. Equine Vet J 48 (4), 442-450 PubMed.
• Lopes C, Luna S P, Rosa, A C et al (2016) Antinociceptive effects of methadone combined with detomidine or acepromazine in horses. Equine Vet J 48 (5), 613-618 PubMed.
• Jago R C, Corletto F & Wright I M (2015) Peri-anaesthetic complications in an equine referral hospital: Risk factors for post anaesthetic colic. Equine Vet J 47 (6), 635-640 PubMed.
• Gozalo-Marcilla M, Gasthuys F & Schauvliege S (2015) Partial intravenous anaesthesia in the horse: a review of intravenous agents used to supplement equine inhalation anaesthesia. Part 2: opioids and alpha-2 adrenoceptor agonists. Vet Anaesth Analg 42 (1), 1-16 PubMed.
• Martin-Flores M, Campoy L, Kinsley M A, Mohammed H O, Gleed R D & Cheetham J (2014) Analgesic and gastrointestinal effects of epidural morphine in horses after laparoscopic cryptorchidectomy under general anesthesia. Vet Anaesth Analg 41 (4), 430-437 PubMed.
• Oliveira F A, Pignaton W, Teixeira-Neto F J, de Queiroz-Neto A et al (2014) Antinociceptive and behavioral effects of methadone alone or in combination with detomidine in conscious horses. J Equine Vet Sci 34, 380-386 VetMedResource.
• van Loon J P, Menke E S, L'ami J J et al (2013) Analgesic and anti-hyperalgesic effects of epidural morphine in an equine LPS-induced acute synovitis model. Vet J 193 (2), 464-470 PubMed.
• Senior J M (2013) Morbidity, mortality and risk of general anesthesia in horses. Vet Clin Equine 29 (1), 1–18 PubMed.
• Figueiredo J P, Muir W W & Sams R (2012) Cardiorespiratory, gastrointestinal, and analgesic effects of morphine sulfate in conscious healthy horses. Am J Vet Res 73 (6), 799-808 PubMed.
• Michou J & Leece E (2012) Sedation and analgesia in the standing horse 2. Local anaesthesia and analgesia techniques. In Pract 34 (10), 578-587.
• Sano H, Martin-Flores M, Santos L C, Cheetham J, Araos J D & Gleed R D (2011) Effects of epidural morphine on gastrointestinal transit in unmedicated horses. Vet Anaesth Analg 38 (2), 121-126 PubMed.
• Freitas G C, Carregaro A B, Gehrcke M I et al (2011) Epidural analgesia with morphine or buprenorphine in ponies with lipopolysaccharide (LPS)-induced carpal synovitis. Can J Vet Res 75 (2), 141-146 PubMed.
• Clutton R E (2010) Opiod analgesia in horse. Vet Clin North Am Equine Pract 26 (3), 493-514 PubMed.
• Gozalo-Marcilla M, de Oliveira A R, Fonseca M W et al (2010) Sedative and antinocieptive effects of different detomidine constant rate infusions, with or without methadone in standing horses. Equine Vet J 51 (4), 530-536 PubMed.
• Clark L, Clutton R E, Blissitt K J & Chase-Topping M E (2008) The effects of morphine on the recovery of horses from halothane anaesthesia. Vet Anaesth Analg 35 (3), 22-29 PubMed.
• Boscan P, Van Hoogmoed L M, Farver T B & Snyder J R (2006) Evaluation of the effects of the opioid agonist morphine on gastrointestinal tract function in horses. Am J Vet Res 67 (6), 992-997 PubMed.
• Love E J, Lane J G & Murison P J (2006) Morphine administration in horses anaesthetized for upper respiratory tract surgery. Vet Anaesth Analg 33, 179-188.
• Clark L, Clutton R E, Blissitt K J & Chase-Topping M E (2005) Effects of peri-operative morphine administration during halothane anaesthesia in horses. Vet Anaesth Analg 32 (1), 10-15 PubMed.
• Senior J M, Pinchbeck G, Dugdale A H & Clegg P D (2004) A retrospective study of the risk factors and prevalence of colic in horses after orthopaedic surgery. Vet Rec 155 (11), 321–325 PubMed.
• Cohen N D, Lester G D, Sanchez L C, Merritt A M & Roussel A J Jr (2004) Evaluation of risk factors associated with development of postoperative ileus in horses. JAVMA 225 (7), 1070-1078 PubMed.
• Sellon D C, Roberts M C, Blikslager A T, Ulibarri C & Papich M G (2004) Effects of continuous-rate infusion of butorphanol on physiologic and outcome variables in horses after celiotomy. J Vet Intern Med 18 (4), 555-563 PubMed.
• Mircica E, Clutton R E, Kyles K W & Blissitt K J (2003) Problems associated with perioperative morphine in horses: a retrospective case analysis. Vet Anaesth Analg 30 (3),147-155 PubMed.
• Maxwell L K, Thomasy S M, Slovis N & Kollias-Baker C (2003) Pharmacokinetics of fentanyl following intravenous and transdermal administration in horses. Equine Vet J 35 (5), 484-490 PubMed.
• Steffey E P, Eisele J H & Baggot J D (2003) Interactions of morphine and isoflurane in horses. Am J Vet Res 64 (2), 166-175 PubMed.
• Olbrich V H & Mosing M (2003) A comparison of the analgesic effects of caudal epidural methadone and lidocaine in the horse. Vet Anaesth Analg 30 (3), 156-164 PubMed.
• Bennett R C & Steffey E P (2002) Use of opioids for pain and anesthetic management in horses. Vet Clin North Am Equine Pract 18 (1), 47-60 PubMed. 
• Goodrich L R, Nixon A J, Fubini S L et al (2002) Epidural morphine and detomidine decreases postoperative hindlimb lameness in horses after bilateral stifle arthroscopy. Vet Surg 31 (3), 232-239 PubMed.
• Sysel A M, Pleasant R S, Jacobson J D et al (1996) Efficacy of an epidural combination of morphine and detomidine in alleviating experimentally induced hindlimb lameness in horses. Vet Surg 25 (6), 511-518 PubMed.
• Roger T, Bardon T & Ruckebusch Y (1994) Comparative effects of mu and kappa opiate agonists on the cecocolic motility in the pony. Can J Vet Res 58 (3), 163-166 PubMed.
• Valverde A, Little C B, Dyson D H & Motter C H (1990) Use of epidural morphine to relieve pain in a horse. Can Vet J 31 (3), 211-212 PubMed.
• Dodman N H, Shuster L, Court M H & Dixon R (1987) Investigation into the use of narcotic antagonists in the treatment of a stereotypic behavior pattern (crib-biting) in the horse. Am J Vet Res 48 (2), 311-319 PubMed.
• Kalpravidh M, Lumb W V, Wright M & Heath R B (1984) Analgesic effects of butorphanol in horses: dose-response studies. Am J Vet Res 45 (2), 211-216 PubMed.
• Kalpravidh M, Lumb W V, Wright M & Heath R B (1984) Effects of butorphanol, flunixin, levorphanol, morphine, and xylazine in ponies. Am J Vet Res 45 (2), 217-223 PubMed.​