Table 6.

Commonly used opioid analgesics in cats, with suggested doses and routes of administration^* (continued on page 16)

Degree of acute pain management (examples)	Drug	Dose (mg/kg or µg/kg where indicated) and potential routes of administration	Duration of analgesia	Comments
Pure µ-opioid receptor agonist
Moderate to severe pain (orthopaedic surgery, abdominal exploratory, cystotomy, ovariohysterectomy, neuropathic pain, thoracotomy, large or multiple mass removal[s], pancreatitis, and those listed below for other classes of opioid)	Morphine	✜ 0.1-0.2 IV, IM or epidural	✜ Up to 6 h for IV or IM ✜ Up to 12-24 h for epidural	Caution with histamine release when administered IV. Administer slowly IV. Vomiting and nausea commonly occur, particularly with IM or SC route. Urinary retention may be observed following epidural administration
	Hydromorphone	✜ 0.025-0.1 IV or IM	✜ Up to 6 h for IV or IM	Vomiting and nausea commonly occur, particularly with IM or SC route. Opioid-induced hyperthermia occurs more frequently with the use of high doses of hydromorphone, either alone or in combination with other anaesthetic drugs. Increases in temperature are generally mild to moderate and are self-limiting (<4 h)
	Meperidine (pethidine)	✜ 5-10 IM	✜ 1-2 h	Should not be administered IV due to histamine release with potential skin reactions and hypotension. Duration of analgesia is short-lived and dose-dependent. Licensed for IM injection in European countries, including the UK
	Methadone	✜ 0.2-0.6 IV, IM or OTM	✜ Up to 6 h	Has NMDA receptor antagonistic effect, which may be helpful to treat peripheral and central sensitisation. Minimal vomiting and nausea when compared with hydromorphone and morphine administration. Higher doses are used for OTM administration, whereas lower doses are given IV. Licensed for use in cats in the UK, Canada, Italy and some other European countries
	Fentanyl	✜ 5 µg/kg bolus followed by infusion from 3-20 µg/kg/h IV ✜ 25 µg/h transdermal patch	✜ Provides analgesia for the duration of the infusion only ✜ Up to 72 h	Low doses are used for analgesia, whereas high doses may also provide some (15-20%) anaesthetic-sparing effect with potential simultaneous sympathetic activation. Plasma concentrations are highly variable with use of transdermal patches (human formulations) of fentanyl in cats. Analgesia is variable and may be suboptimal. Apply fentanyl patch 18-24 h prior to noxious stimuli
	Remifentanil	✜ 6-40 µg/kg/h IV	✜ Provides analgesia for the duration of the infusion only	Low doses are used for analgesia, whereas high doses may also provide some (15-20%) anaesthetic-sparing effect with potential simultaneous sympathetic activation. Loading doses of remifentanil are not required if started 15 mins prior to surgical stimulation. May contribute to opioid-induced hyperalgesia; however, this has not been confirmed in cats. Does not require hepatic metabolism; undergoes degradation via non-specific plasma and tissue cholinesterase
Mild to moderate pain (castration, ovariohysterectomy, small mass removal, osteoarthritis, endoscopy, urinary catheterisation, and those listed below for agonist-antagonist opioids)	B uprenorphine – standard formulation (0.3 mg/ml)	✜ 0.02-0.04 IV, IM or OTM	✜ Up to 8 h	SC administration using the standard formulation is not recommended. There is no histamine release, vomiting or nausea after the administration of buprenorphine. Possible OTM administration for ‘hands-off’ analgesia, especially as part of multimodal analgesia. Both formulations when combined with an NSAID provide adequate analgesia for dental and neutering procedures. Not sufficient for acute pain relief following major orthopaedic procedures, especially when used as a sole analgesic. Licensed for use in cats in the UK, Canada, Australia and other countries. The high-concentration formulation is FDA-approved in the USA
	Buprenorphine – high concentration (1.8 mg/ml)	✜ 0.24 SC	✜ Up to 24 h
Mixed µ-opioid receptor antagonist/K-agonist
Mild pain or to help facilitate diagnostic procedures and sedation (phlebotomy, cystocentesis, physical examination, fine-needle aspiration)	Butorphanol	✜ 0.2-0.4 IV or IM	✜ 1-2 h	Provides appropriate analgesia when used in a multimodal protocol for orchiectomy. Does not provide adequate analgesia when used as a sole analgesic for routine surgical procedures. Higher doses do not provide additional analgesic benefit. Large inter-patient variability in analgesia has been reported. May be a better alternative than naloxone when reversing (0.02-0.05 mg/kg IV) pure opioid receptor agonists due to its ability to preserve some antinociception
	Nalbuphine	✜ 0.15-0.3 IV or IM	✜ Not yet determined	May be a substitute for when butorphanol is unavailable. Minimal research has been performed to evaluate effectiveness for acute pain management. Dose to provide analgesia is yet to be determined
µ-opioid receptor antagonist (reversal agent)
	Naloxone	✜ 0.01-0.04 IV diluted to 5 ml and given slowly to effect	NA	Fast administration of 0.04 mg/kg IV can produce depressed mentation and dullness (B Simon, personal observations). Administer slowly in small increments every 60 s and to effect. Analgesia will be reversed and other analgesic techniques and drugs must be considered if pain is an issue

^*Doses, drugs and routes of administration presented in this table are suggestions and not exhaustive. Individual variability should be appreciated and opioids are better administered as part of multimodal analgesia. The analgesic effects may be variable depending on the type of pain, age, breed, surgical procedure, anaesthetic duration and route of administration. Clinical judgement is important. Veterinarians should consult drug labels if a veterinary product is available

NA = not applicable; IV = intravenous; IM = intramuscular; SC = subcutaneous; OTM = oral transmucosal; NSAID = non-steroidal anti-inflammatory drug; NMDA = N-methyl D-aspartate; FDA = Food and Drug Administration