Transdermal Medications: How They Work and How to Apply Them

By Malcolm Weir, DVM, MSc, MPH; Ernest Ward, DVM

My veterinarian has suggested I use a transdermal medication for my pet. What is it?

Transdermal means the application of a medicine or drug through the skin. In the simplest terms, a drug is placed on top of the skin, where it is absorbed into the bloodstream. Transdermal medications have many advantages, the main one being ease of application. Medications that can be absorbed through the skin bypass the need for pills or liquids, which can be a challenge to administer to some pets. In addition, because they are absorbed directly into the bloodstream, and avoid initially passing through the intestinal tract and liver, drugs that are administered transdermally may be more effective or work faster than some oral medications, allowing for better treatments.

Can any medication be made into a transdermal form?

Unfortunately, no. Relatively few drugs are currently available for transdermal application. The skin is designed as a protective barrier, and crossing that barrier is challenging, especially in the consistent and controlled manner necessary for drug administration. Complicating matters for dogs and cats are the presence of fur and variations in their skin. Most transdermal drugs must be dissolved in a gel or suspended in a patch. As techniques for transdermal administration improve, we expect to see an increase in the choices we have for veterinary medications.

What medications are available for transdermal administration?

Technology changes rapidly, so it is best to ask your veterinarian. Your veterinarian will often consult with a compounding pharmacist to determine if your pet’s medication can be made into a transdermal gel or patch. Currently, several common medications have been used successfully as a transdermal application. They include methimazole (to treat hyperthyroidism), amitriptyline (to manage behavioral issues and provide pain relief), fentanyl (for pain relief), and several heartworm, flea, and tick preventives.

"Technology changes rapidly, so it is best to ask your veterinarian."

The way each drug works and is absorbed transdermally varies considerably. Hair, differences in skin, skin infections, effects of illness, or hydration status, can all influence the body’s ability to use a drug applied on the skin. If your pet is placed on a transdermal medication and fails to improve, the drug may need to be administered orally.

Are there any risks with transdermal medications?

The biggest risk with a transdermal medication is that it could be insufficiently absorbed, resulting in inadequate drug levels and treatment. Other potential issues include:

  • the pet inadvertently licking or orally consuming the medication, causing gastrointestinal (GI) upset or other more serious side effects
  • local irritation or hypersensitivity to the transdermal formulation
  • allergic reactions
  • accidental human exposure
  • alterations of the original formulation, resulting in too much or too little of the active ingredient being delivered

When using transdermal formulations, your veterinarian and pharmacist will work closely together to ensure that your pet receives the proper amount of medication.

How do I administer a transdermal medication to my pet?

The ideal application site for transdermal medications is one that has minimal hair and that cannot be easily licked or rubbed. Some pets may require shaving to allow for better administration. The inside of the outer ear flap (not inside the ear canal) is an excellent area for many pets.

Most transdermal medications will come in pre-dosed syringes or in gel form that you can draw into a syringe to the prescribed dosage. You should wear protective gloves when handling transdermal medications. Remember that these drugs have been formulated to cross the skin barrier, so you can potentially be exposed if you come in contact with them.

"To avoid being exposed to the drug, wear protective gloves when
handling transdermal medications."

If your pet is prescribed a transdermal gel, you may need someone to assist you, especially at first. Here are some tips for administration:

  • have your helper gently hold your pet and distract it by petting or offering a toy or treat.
  • make sure the area on the ear where you are going to apply the medication is clean and dry.
  • slowly and deliberately apply (“squirt”) the correct amount of medication onto your pet’s ear.
  • if necessary, gently rub the gel into the skin to facilitate absorption using your gloved fingertip or the tip of the syringe. It is important not to leave a clump, glob, or thick area of medication on the ear because it will be easily dislodged if your pet shakes its head after administration.

After you have administered the medication, try to distract your pet from scratching or rubbing the ear for a few minutes. This can best be accomplished by feeding your pet, taking your pet for a walk, or playing together immediately afterward. Such a reward also functions as an excellent training technique that can help teach your pet not to fear the application of medications.

If your pet is prescribed a transdermal medication patch, your veterinarian will often shave an area to affix the patch. Many patches will last for several days, and the patch may have a protective wrap or bandage over it. Your veterinarian will explain the proper care, duration, and monitoring of transdermal patches. It is very important that your pet not be allowed to lick or swallow the patch. Serious side effects may result if a pet inadvertently consumes a patch, especially one containing fentanyl, a pain-relief drug.

"Serious side effects may result if a pet consumes a transdermal patch, especially one containing fentanyl, a pain-relief drug."

Ask your veterinarian if you have any questions or concerns about your pet’s transdermal medication.

What is my takeaway message?

Transdermal medications are an excellent alternative for some veterinary drugs, and options are increasing each year. Talk to your veterinarian to determine if a transdermal medication is appropriate for your pet.

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