Fentanyl

What is this medication for?

• Peri-op/CRI analgesia (injectable); post-op pain control up to 4 days (Recuvyra, dogs) or via patch

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

• Dosing is in micrograms/kg in most references — always double-check mcg vs mg to avoid catastrophic 1000x errors.
• Patch absorption is highly interpatient-variable — always have injectable rescue analgesia available when relying on a patch as primary analgesic.
• Never apply external heat (heating pads) over a fentanyl patch — fatal overdoses have resulted from increased absorption.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• Respiratory/CNS/circulatory depression (unusually slow heart rate) (Dose-related)
• Rash at patch site, urine retention, constipation, dysphoria/agitation (Reported)
• Excitement, restlessness, unusually fast heart rate (Common at analgesic doses)

Worth a phone call to your veterinarian

• Any new symptom that worries you.

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Trouble breathing, collapse, seizures, severe vomiting or diarrhea, facial swelling, or pale gums

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Known hypersensitivity
• has or has had: Geriatric/debilitated/pre-existing respiratory disease; febrile patients (↑ absorption from patch); do not expose patch to external heat sources (fatal over-absorption reported)

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Azole antifungals, macrolides (erythromycin, clarithromycin)
  • MAO inhibitors (amitraz, selegiline)
  • Phenobarbital, phenytoin, rifampin
  • Tricyclic antidepressants
  • Other CNS depressants

Storage and handling

Room temperature, protect from light, Schedule II — store in locked compartment. Injectable pH 5.2 — can add bicarbonate (1:10–1:20) to raise pH to 8.0 and reduce injection pain, use immediately after mixing.

Good to know

• Dosing is in micrograms/kg in most references — always double-check mcg vs mg to avoid catastrophic 1000x errors.
• DO NOT CUT transdermal patches — alters the drug-release membrane and can cause dangerous rapid release; consult a pharmacist if a partial dose is needed.
• Patch absorption is highly interpatient-variable — always have injectable rescue analgesia available when relying on a patch as primary analgesic.
• Never apply external heat (heating pads) over a fentanyl patch — fatal overdoses have resulted from increased absorption.