Fentanyl

Overview

Fentanyl is a highly potent synthetic opioid analgesic used in dogs and cats for perioperative analgesia and the control of severe pain. “Fentanyl is a highly potent opioid analgesic, reported to be approximately 50–100 times more potent than morphine.”

It has a rapid onset of action following IV administration and a short duration of effect (typically 10–20 minutes), making it well suited for intraoperative analgesia via intermittent boluses or continuous rate infusion (CRI). With prolonged administration or high doses, its duration may increase due to tissue accumulation.

Transdermal fentanyl patches provide a noninvasive option for sustained analgesia, reported as up to 72 hours and, in some studies, about 3 days in dogs and up to 5 days in cats. However, onset is delayed (12 to 24 hours in dogs and 6 to 24 hours in cats), and plasma concentrations can be variable, so additional analgesia is required during the initial period.

Mechanism of Action (MOA): Fentanyl is a pure mu (μ) opioid receptor agonist that produces profound analgesia by modulating pain perception within the central nervous system. It also contributes to sedation and reduces anesthetic requirements when used in combination with other agents.

Indications

Fentanyl is used in dogs and cats as a potent opioid analgesic for the management of severe pain, particularly in perioperative and critical care settings.

• Intraoperative analgesia: Widely used during anaesthesia to provide profound analgesia as part of a balanced anaesthesia protocol, reducing the required doses of other agents.
• Postoperative pain management: Administered as a continuous rate infusion (CRI) at lower doses to maintain analgesia after surgery.
• Chronic or prolonged pain control: Transdermal patches can provide sustained analgesia for up to 72 hours, and in some reports about 3 days in dogs and up to 5 days in cats, particularly useful in cases requiring ongoing pain management.
• Anesthetic-sparing effect: Used to reduce the required doses of other anesthetic agents, especially in patients with cardiovascular instability or systemic disease.

Dosage (Reference)

Dog

In dogs, fentanyl is primarily used for intraoperative and postoperative analgesia via IV bolus or continuous rate infusion (CRI). It may also be administered via transdermal patches for prolonged pain control.

Cat

In cats, fentanyl is used similarly to dogs for perioperative analgesia, with both bolus dosing and continuous rate infusion. Transdermal patches are commonly used for sustained analgesia.

Warnings & Precautions

Fentanyl is a potent opioid requiring careful monitoring in dogs and cats, particularly due to its effects on respiration and variability in transdermal absorption.

• Respiratory depression: Common during intraoperative use; respiratory function must be closely monitored and facilities for assisted ventilation should be available.
• Bradycardia (IV use): Rapid IV administration may cause marked bradycardia or even asystole; always administer slowly. Anticholinergics (e.g., atropine) may be required.
• Short duration (IV): Analgesic effect is brief (10–20 minutes), requiring repeated dosing or CRI for sustained analgesia.
• Prolonged effect with accumulation: High doses or prolonged infusions can lead to drug accumulation and extended duration of action.
• Delayed onset (patches): Therapeutic levels are delayed (12 to 24 hours in dogs, 6 to 24 hours in cats); alternative analgesia must be provided during this period.
• Heat exposure (critical): External heat sources (e.g., heating pads) can significantly increase fentanyl absorption from patches, leading to overdose.
• Home-use safety: If animals are discharged with patches, owners must be warned about accidental ingestion by humans or other animals.
• Contraindications: Contraindicated in patients with known hypersensitivity to fentanyl, other opioids, or any component of the product, including the patch adhesive.
• Controlled drug status: In the United States fentanyl is a Schedule II controlled substance: store it in a securely locked, substantially constructed area, and follow applicable rules for disposal of unused or wasted drug.
• Name confusion: Do not confuse fentanyl with alfentanil, carfentanil, remifentanil or sufentanil.
• Patch disposal: Used patches retain substantial fentanyl and can poison people, especially children, if they reach household waste. Dispose of them promptly through a drug take-back programme.
• Dose units and double-checking: All fentanyl dosages listed here are extra-label. Do not confuse doses in µg/kg with mg/kg — infusion rates may be written as mg/kg/hour, µg/kg/hour or µg/kg/minute. Always double-check the calculated dose and the resulting volume, and have naloxone readily available when giving fentanyl.
• Patch content is not the delivery rate: Do not confuse the total fentanyl content of a patch with its hourly delivery rate — a 75 µg/h patch contains around 7.65 mg of fentanyl.
• Patients needing extra caution: Use with extreme caution where CNS or respiratory depression already exists, or alongside other CNS, respiratory or cardiac depressants. Use cautiously in geriatric, ill or debilitated patients and in those with gastrointestinal obstruction, pre-existing respiratory disease, or renal or hepatic impairment.
• Scorpion envenomation: Opioids are contraindicated in patients stung by scorpions (for example Centruroides species), as they may potentiate the venom’s effects.
• Fever and patch absorption: A rising body temperature increases fentanyl absorption. Consider removing the patch if the patient develops a fever; if it is left in place, monitor closely for adverse effects.
• Do not cut patches: Prescribe an appropriately sized patch rather than cutting one down; consult a veterinary pharmacist before cutting any patch.
• Mydriasis in cats: Opioids may cause mydriasis in cats. Approach affected cats slowly so as not to startle them, and keep them out of bright light while the pupils are dilated.
• Pregnancy and the neonate: Opioids cross the placenta, and prolonged use during pregnancy may cause neonatal withdrawal. Embryocidal effects and reduced pup survival have been reported in laboratory rats.
• Patch application: Clip the site, wipe it gently with a damp cloth using water only, and allow it to dry completely. Do not use alcohol or surgical scrub, as they may alter absorption. Hold the patch in place for 2 to 3 minutes, ensure it is fully adhered — fentanyl is not absorbed properly otherwise — and label it with the patch size and the date and time of application.
• Tolerance and dependence: Tolerance and physical dependence may develop with repeated administration.
• Fertility and nursing: Fentanyl may reduce fertility in males and females, and most opioid agonist analgesics are excreted into milk.

Drug Interactions

Fentanyl reduces the dose requirements of concurrently administered anaesthetics, but it also has clinically important additive CNS, respiratory and cardiovascular effects and several metabolic and serotonergic interactions.

• General anesthetic agents: Fentanyl reduces the required doses of induction and inhalant anaesthetics, contributing to balanced anaesthesia.
• Sedatives and CNS depressants: Additive CNS and respiratory depression may occur when combined with other sedatives or anesthetic drugs; careful monitoring is required.
• Cardiovascular-compromised patients: Can be used to minimize the need for higher doses of other anesthetic agents in patients with systemic disease or cardiovascular instability.
• MAO inhibitors (including amitraz): Avoid combining fentanyl with MAO inhibitors such as amitraz or selegiline; concurrent use risks anxiety, confusion, hypotension, respiratory depression, cyanosis and coma. Withhold the MAO inhibitor for 14 days before giving an opioid.
• Serotonergic drugs and tramadol: Concurrent use with serotonergic agents such as fluoxetine or ondansetron may increase the risk of serotonin syndrome; avoid combining these drugs. Tramadol additionally increases the risk of seizures, urine retention and constipation.
• Antifungal interactions: Azole antifungals such as fluconazole, itraconazole and ketoconazole may prolong fentanyl’s effects and increase the risk of respiratory depression, although one canine study found ketoconazole did not significantly alter fentanyl elimination.
• Enzyme induction: Hepatic enzyme inducers such as griseofulvin, mitotane or phenobarbital may reduce fentanyl concentrations; monitor the analgesic response and adjust the dose accordingly.
• Intrathecal iohexol: Withhold opioids for 48 hours before and 24 hours after intrathecal iohexol, as concurrent use may increase the risk of seizures.
• Drugs that slow fentanyl clearance: Diltiazem, cimetidine and macrolides such as clarithromycin or erythromycin may increase fentanyl’s half-life and reduce its clearance, prolonging its effects and increasing the risk of respiratory depression.
• Cardiovascular drugs: ACE inhibitors, angiotensin-receptor blockers, beta-blockers and calcium-channel blockers may increase the risk of bradycardia and hypotension when given with fentanyl.
• Desmopressin: Fentanyl may increase oral desmopressin concentrations and the risk of water intoxication or hyponatraemia, which can progress to seizures, coma, respiratory arrest and death; monitor electrolytes and renal function.

Side Effects & Overdose

Side Effects

Adverse effects of fentanyl in dogs and cats are primarily related to its opioid activity and are most commonly observed during or shortly after administration.

• Respiratory depression: The most significant effect, especially during anesthesia or with higher doses; requires close monitoring.
• Bradycardia: Commonly observed following administration, particularly with IV use.
• Severe bradycardia or asystole: May occur with rapid IV injection.
• Postoperative dysphoria in dogs: Dysphoria has been reported in around 25% of dogs receiving a postoperative fentanyl infusion; acepromazine or another mild tranquilliser may settle it.
• Dose-related effects: Other dose-related effects include sedation, bradypnoea, hypercapnia and hypotension. Hypothermia is possible in dogs.
• Patch-site reactions: Transdermal patches may cause a rash at the application site; remove the patch and choose a different site if a further patch is needed. Urine retention and constipation may also occur.
• Excitement in cats: Cats may become excited after administration, especially when fentanyl is given by the IV route.

Overdose

Overdose of fentanyl can result in profound opioid-related toxicity, primarily affecting the respiratory and central nervous systems.

• Severe respiratory depression: Leading to hypoventilation or apnea, which may be life-threatening.
• Marked CNS depression: Deep sedation, unresponsiveness, or coma may occur.
• Cardiovascular effects: Pronounced bradycardia and potential circulatory compromise.
• Management: Requires immediate supportive care, including airway management, oxygen supplementation and assisted ventilation if needed. Naloxone is the reversal agent of choice; massive overdoses may need repeated doses, and patients must be observed closely, as naloxone’s effects sometimes diminish before fentanyl levels become subtoxic.
• Other overdose signs: Newborn animals may be more susceptible. Other signs of overdose include hypersalivation, ataxia, hypothermia, diarrhoea, cardiovascular collapse, tremors, neck rigidity and seizures.
• Naloxone: If adverse effects occur in a patient wearing a patch, remove the patch first. If reversal is required, naloxone hydrochloride 0.04 mg/kg may be given.

Key Notes

Practical clinical points to optimize the use of fentanyl in dogs and cats:

• Balanced anesthesia role: Incorporating fentanyl contributes to balanced anaesthesia and reduces the required doses of other anaesthetic agents.
• Flexible administration: Can be administered as intermittent boluses or continuous infusion depending on procedural needs.
• High lipid solubility: Contributes to rapid onset and redistribution, influencing its pharmacokinetic profile.
• Tissue saturation effect: Prolonged administration leads to drug accumulation in tissues, extending duration of action beyond initial expectations.
• Patch use as adjunct: Transdermal fentanyl is best used alongside other analgesics rather than as sole therapy.
• Variable patch response: Clinical effect from transdermal systems may differ significantly between patients.
• Canine half-life: Reported terminal elimination half-life after intravenous administration varies considerably between studies, ranging from approximately 45 minutes to 2–6 hours.
• Patch duration: Patches have released effective fentanyl concentrations for up to 5 days in cats and about 3 days in dogs.
• Feline patch delivery: Delivery from a feline patch is variable: a 25 µg/h patch provides roughly 10 µg/h in practice. Target around 2 to 2.5 µg/kg/h (range 1 to 5) and select the patch size that comes closest for the cat’s weight.
• Oral route: Swallowed fentanyl tablets are unlikely to give useful analgesia in dogs or cats due to a high first-pass metabolism.
• Safety margin and ventilation: In spontaneously breathing dogs, doses as high as 300 times the recommended dose were not lethal in one report. Fentanyl still causes dose-related respiratory depression, so monitor respiration and ensure the means to provide positive-pressure ventilation are available.
• Monitoring: Monitor analgesic efficacy, heart rate, blood pressure, respiratory rate, pulse oximetry and end-tidal CO₂, together with CNS effects such as sedation, dysphoria or excitement.
• Amylase and lipase: Fentanyl may raise plasma amylase and lipase for up to 24 hours after administration by increasing biliary-tract pressure.
• Storage: Store the injection protected from light at controlled room temperature. Store patches below 25°C, do not freeze them, and apply immediately after opening the sealed package.
• Subcutaneous injection: The pH of the injectable solution can elicit pain on subcutaneous administration; adding bicarbonate at 1:10 or 1:20 dilution has been described to reduce it.
• Buccal products: Oral transmucosal (buccal) fentanyl products are human formulations and have not been adequately tested in animals.
• Diluents and compatibility: The injection is compatible with normal saline and 5% dextrose, and has reported compatibility with clonidine, bupivacaine, ropivacaine and ketamine.
• Canine rates by condition: Reported canine infusion rates sit within the published band: 3–5 µg/kg/hour after traumatic brain injury and 2–5 µg/kg/hour for acute pancreatitis. A pain-control infusion is started with a 3–5 µg/kg IV loading dose, then 5 µg/kg/hour, increased to 10 µg/kg/hour if pain persists.
• Feline rates by condition: Reported feline infusion rates sit within the published band: 2–4 µg/kg/hour for acute pancreatitis. A pain-control infusion is started with a 3–5 µg/kg IV loading dose, then 2–5 µg/kg/hour, increased to 7 µg/kg/hour if needed.