Major Uses

Fentanyl is a narcotic pain reliever in the same class of drugs as opium and morphine. It is used as a transdermal patch to provide continuous pain relief to pets recovering from surgery or trauma or experiencing extreme pain due to other causes. It is also available by injection to cause sedation. The injection is approved for use in animals by the Food and Drug Administration, but the patch is not approved for use in animals. However, it is prescribed legally as an extra-label drug by veterinarians.

Because of its high abuse and addictive potential, fentanyl (and similar drugs) are Class II controlled substances and can only be prescribed by veterinarians with an active Drug Enforcement Agency license.

Common Precautions

Fentanyl is associated with numerous side effects, some of which are potentially serious. One of the most serious is respiratory depression, a condition that can lead to death if not treated promptly. Exposing the patch to heat sources, such as electric blankets and heated water beds, increases this risk. This medication should be used cautiously in animals with fever.

Skin reactions to the adhesive backing on the patch are not uncommon. These reactions are usually minor and should resolve following removal of the patch and application of topical cortisone. Some animals being treated with fentanyl may experience excessive appetite. Nausea and reduced appetite are also possible. A wobbly gait may be a sign of sedation associated with patch use.

If swallowed, the fentanyl patch may be toxic to animals. The patches may also be toxic to small children.

This drug should be used cautiously in combination with other drugs that cause sedation, such as other pain relievers and antihistamines. Fentanyl should not be used in animals undergoing treatment with L-Deprenyl. This drug may also interact with barbiturates and monoamine oxidase inhibitors.

Fentanyl should not be used in animals with a known allergy or hypersensitivity to the drug. Any known or suspected ingestion or overdose should be reported to a veterinarian immediately.


Fentanyl patches should be stored at room temperature, away from excessive heat and moisture, and out of the reach of children and animals. Used patches should be folded in half so that the sticky sides are together. A veterinarian can provide more detailed disposal information.


Fentanyl is available in 12.5 mcg per hour, 25 mcg per hour, 50 mcg per hour, 75 mcg per hour, and 100 mcg per hour patches. It is also available in injectable form with another drug called droperidol. Each ampule contains 0.05 mg fentanyl and 2.5 mg droperidol per ml.

Injectable fentanyl is only for use in a hospital setting. The fentanyl patch is for home use. Dose is based on weight. Animals weighing less than 20 pounds (10 kg) should use a 2.5 mg patch. Animals weighing 20 to 40 pounds (10 to 20 kg) should use the 5 mg patch. Animals weighing 40 to 60 pounds (20 to 30 kg) should use the 7.5 mg patch, and animals over 60 pounds (30 kg) should use the 10 mg patch.

When treating very small animals, only 1/2 of the patch should be used. It is crucial that the patch NOT be cut. Taping over 1/2 of the patch will reduce the dose. Patches are only effective for three days and should then be removed or replaced.

Doses may vary in different species, when the drug is given by a different route or concurrently with other medications, and with regards to a patient's age, breed, and health status. A veterinarian's dosing instructions and/or those printed on the medication label should be followed closely. Only one patch should be used on an animal at a time.

This information is for general reference only and should not be used for the diagnosis or treatment of any condition of your pet. It's intended as a general reference, this information may not include all possible uses, precautions, directions, reactions (including allergic), drug interactions, or withdrawal times. Always consult your local veterinarian and have your pet examined for any advice concerning the diagnosis and treatment of your pet, including which products and doses are most appropriate. Any trademarks are the property of their respective owners. VetDepot is not a pharmacy. All prescription products are dispensed by our Pharmacy Partner. Article last updated 2/2014.