Fentanyl

Fentanyl is a potent synthetic opioid used for severe pain and as an analgesic component of balanced anesthesia. Veterinary use is greatest in dogs and cats, including critically ill or postoperative patients. Injectable and transdermal formulations provide different exposure patterns, with brief effects after some injections and more prolonged delivery from skin-based systems.

Group

Potent mu-opioid agonist

Mechanism of Action

Fentanyl activates mu-opioid receptors coupled to inhibitory G proteins in central and peripheral pain pathways. Reduced presynaptic calcium entry and increased postsynaptic potassium conductance suppress neurotransmitter release and neuronal excitation. This decreases transmission and perception of nociceptive signals and reduces anesthetic requirements. High lipid solubility supports rapid access to central receptor sites after systemic administration.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Pain-control infusion — Loading	2–5 mcg/kg	IV	Once	Loading dose, then use the associated infusion titrated to effect.
Pain-control infusion — Maintenance	2–5 mcg/kg/hour	IV	Continuous	Titrate to analgesia and respiratory status.
Surgical analgesia infusion	10–45 mcg/kg/hour	IV	Continuous	Anaesthetic protocol requiring continuous monitoring and ventilation support as necessary.
Perioperative infusion - study alternative — Loading	10 mcg/kg	IV	Once	Loading dose before 10 mcg/kg/hour during general anaesthesia.
Perioperative infusion - study alternative — Maintenance	10 mcg/kg/hour	IV	Continuous
Epidural analgesia	4 mcg/kg	EPIDURAL	Once	Use a neuraxially suitable preservative-free preparation. Source onset less than 10 minutes and duration about 30 minutes; its printed dilution volume is ambiguous and is not inferred here.
Intermittent analgesia - alternative	5–10 mcg/kg	IV, IM, SC	Every 2 h	Source intermittent analgesia alternative; titrate with respiratory monitoring and a route-appropriate preparation.
Severe emergency multimodal analgesia	Reference only	IV		Specialist multimodal protocol: fentanyl 10-50 mcg/kg IV titrated to effect, with the effective amount informing the hourly CRI; ketamine 4 mg/kg bolus and lidocaine 2-4 mg/kg bolus followed by 2-4 mg/kg/hour are separate components in the cited protocol. Account for any other local anaesthetic already given. The upper fentanyl bolus is not a routine starting dose; control ventilation and urgently treat the painful cause.
Transdermal analgesia	Reference only	TOPICAL		Source whole-patch delivery table: dogs 5-10 kg 25 mcg/hour; 10-20 kg 50 mcg/hour; 20-30 kg 75 mcg/hour; above 30 kg 100 mcg/hour. Small dogs under 5 kg and cats are described with 12.5 or 25 mcg/hour patches. Boundaries overlap and absorbed delivery varies; this is not a validated weight-band tablet table. Do not cut patches.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Pain-control infusion — Loading	1–3 mcg/kg	IV	Once	Loading dose, then use the associated infusion titrated to effect.
Pain-control infusion — Maintenance	1–4 mcg/kg/hour	IV	Continuous	Titrate to analgesia and respiratory status.
Surgical analgesia infusion	10–30 mcg/kg/hour	IV	Continuous	Anaesthetic protocol requiring continuous monitoring and ventilation support as necessary.
Perioperative infusion - alternative — Loading	2–3 mcg/kg	IV	Once	Loading before 2-3 mcg/kg/hour infusion.
Perioperative infusion - alternative — Maintenance	2–3 mcg/kg/hour	IV	Continuous
Epidural analgesia	4 mcg/kg	EPIDURAL	Once	Use a neuraxially suitable preservative-free preparation. Source onset less than 10 minutes and duration about 30 minutes; its printed dilution volume is ambiguous and is not inferred here.
Intermittent analgesia - alternative	5–10 mcg/kg	IV, IM, SC	Every 2 h	Source intermittent analgesia alternative; titrate with respiratory monitoring and a route-appropriate preparation.
Severe emergency multimodal analgesia	Reference only	IV		Specialist multimodal protocol: fentanyl 10-50 mcg/kg IV titrated to effect, with the effective amount informing the hourly CRI; ketamine 4 mg/kg bolus and lidocaine 0.25-1 mg/kg bolus followed by 0.5-2 mg/kg/hour are separate components in the cited protocol. Account for other local anaesthetics and feline lidocaine sensitivity. The upper fentanyl bolus is not a routine starting dose; control ventilation and urgently treat the painful cause.
Transdermal analgesia	Reference only	TOPICAL		Source whole-patch delivery table: dogs 5-10 kg 25 mcg/hour; 10-20 kg 50 mcg/hour; 20-30 kg 75 mcg/hour; above 30 kg 100 mcg/hour. Small dogs under 5 kg and cats are described with 12.5 or 25 mcg/hour patches. Boundaries overlap and absorbed delivery varies; this is not a validated weight-band tablet table. Do not cut patches.
Critical-patient analgesia and heavy sedation	Reference only	IV		Specialist protocol with a local line block: fentanyl 0.5-0.8 mcg/kg/minute (approximately 30-48 mcg/kg/hour), with midazolam 0.1 mg/kg given as a bolus or over one hour if needed. Provide 100% oxygen and monitor end-tidal CO2 or blood gases; assist ventilation when necessary. This is an anaesthetic/sedation protocol, not a routine ward analgesic rate.

Ferrets

Use / stage	Dose	Route	Schedule	Instructions
Perioperative analgesia	5–10 mcg/kg	IV	Once

Rabbits

Use / stage	Dose	Route	Schedule	Instructions
Perioperative analgesia	5–20 mcg/kg	IV	Once	Effect about 30-60 minutes; sedation and respiratory depression can occur.
Transdermal analgesia	Reference only	TOPICAL		Source table lists a 25 mcg/hour patch; another example describes half exposed membrane for a 3 kg rabbit every three days without cutting. These are not a general fixed dose; use a complete specialist protocol.

Sheep

Use / stage	Dose	Route	Schedule	Instructions
Analgesia	5–10 mcg/kg	IV	Once
Transdermal analgesia	Reference only	TOPICAL		Source table gives 1-3 patches of 50 mcg/hour without a resolved weight qualification; not a numeric universal regimen.

Goats

Use / stage	Dose	Route	Schedule	Instructions
Analgesia	5–10 mcg/kg	IV	Once
Transdermal analgesia	Reference only	TOPICAL		Source table lists a 50 mcg/hour patch without a resolved weight qualification; not a numeric universal regimen.

Horses

Use / stage	Dose	Route	Schedule	Instructions
Transdermal analgesia	Reference only	TOPICAL		Source example for a 350-500 kg horse is two 100 mcg/hour patches; duration about two days and onset over six hours. Not an unrestricted per-kg dosing rule.

Pigs

Use / stage	Dose	Route	Schedule	Instructions
Transdermal analgesia	Reference only	TOPICAL		Source example for a 17-25 kg pig is 50-100 mcg/hour patch delivery; patient and formulation dependent.

Indications

Control of severe acute or perioperative pain in dogs and cats.
Analgesic supplementation during balanced general anesthesia.
Treatment of selected severe pain associated with trauma, pancreatitis, major surgery or other critical illness.
Selected sustained analgesia for postoperative or cancer-associated pain using a supported transdermal formulation or specialist protocol.

Contraindications

Avoid uncontrolled respiratory depression without airway/ventilation support.
Do not use a transdermal patch in febrile patients or expose it to external heat.

Interactions

Other sedatives, anaesthetics and opioids intensify respiratory/CNS depression.
MAO inhibitors and other serotonergic agents can increase adverse interaction risk; azoles may increase fentanyl exposure.

Adverse effects

Respiratory depression or apnoea, bradycardia, sedation or dysphoria.
Constipation, urinary retention, hypothermia and possible chest-wall rigidity with rapid high exposure.

Toxicity

Stop delivery, remove any patch, support ventilation and circulation, and use naloxone when indicated. Repeated reversal/monitoring may be needed because absorbed drug can outlast naloxone.

Preparations

Preparation	Form	Strength	Route
Fentanyl injection 50 mcg/mL Fentanyl equivalent supplied as citrate. Verify the exact product supports the selected route; neuraxial use requires a suitable preservative-free presentation.	Injection	50 mcg / mL	IV, IM, SC

Monitoring and Administration

Check mcg versus mg independently: 1 mg = 1000 mcg. Titrate to analgesia and continuously monitor ventilation, oxygenation, heart rate, blood pressure and temperature.

Use a syringe pump for CRI and distinguish per-hour from per-minute doses. Provide airway/ventilation capability for anaesthetic protocols.

Transdermal onset can take 12-24 hours in dogs, 6-24 hours in cats and over 6 hours in horses; bridge with titratable analgesia and taper only while observing response.

Never cut a patch or apply heating pads. Clip without shaving, clean with water rather than alcohol, dry, adhere securely, label application time and prevent licking, chewing or child contact. Used patches retain active drug.

Source patch intervals are approximately three days in dogs, up to five days in cats and two days in horses; marked interpatient variability prevents assuming a fixed absorbed mg/kg dose.