Video: When to Use Transdermal Drugs Like Methimazole

It can be difficult to administer an oral pill or capsule to a cat. I know this only too well, since one of my own cats is just impossible to medicate orally. Other than injections of drugs under the skin or into the muscle, another popular option is to administer the drug topically. 

In this video, veterinary pharmacologist Dr. Dawn Boothe helps you decide whether you should administer drugs transdermally and when this route might work.



Video found on DVM360

Dawn Merton Boothe, DVM, MS, PhD, Dip. ACVIM, Dip. ACVCP 

Department of Anatomy, Physiology, and Pharmacology 

College of Veterinary Medicine Auburn University, Alabama

My Comments
Many cats can be medicated without much difficulty, but some are resistant to handling by their owners so alternative methods of drug delivery must be considered. In these cats, transdermal medication may be the way to go.

Transdermal delivery is a process that involves administering medications through the skin. The drug, dissolved or suspended in some gel or patch, is absorbed through the skin and into the bloodstream. This system of drug delivery is often used in people but has only recently become popular in animals. The most significant advantage of transdermal medications is the ability to administer drugs to cats that cannot or will not take oral medications.

However, there are some disadvantages associated with transdermal drug delivery.

• Applying drugs transdermally on a cat's pinna (ear lobe) has obvious benefits, but absorption of most drugs through the skin is low.
• Although transdermal medications have been employed in human medicine for decades, not as much research has been done in animals (Dr. Boothe is one of the veterinarians who has done much in this regard). Therefore, we don't always know whether a specific drug will actually get absorbed through the skin to have any measurable effect. 
• The amount of a drug absorbed through cats' relatively thick skin is unpredictable and absorption may be erratic.
• Some drugs cannot be made into transdermal formulations because the dose of the medication is too high. Others are too potent and carry a high risk of toxicity. 
• Other potential complications associated with transdermal drug delivery include skin reactions and allergy to the medication. 
• Finally, since cats constantly groom themselves, there is a risk of them ingesting the medication.

As Dr. Boothe mentioned in her video, one drug that is well absorbed when administered transderally is methimazole, the antithyroid drug that is commonly used to control hyperthyroidism in cats. Because we can measure the cat's serum thyroid hormone concentrations after methimazole, we can tell if the transdermal medication is working (or not).  For more information, see my previous blog post on "Treating Cats with Hyperthyroidism: Antithyroid Drugs."