Transdermal Medications

Last Modified: May 6, 2007

Dear OncoLink "Ask The Experts,"

Some animals are very stressed by pilling and oral dosing of meds. The pharmacies will make many compounds in transdermal form but there seems to be very little definitive information on the efficacy of transdermal delivery versus oral. Is transdermal a viable option for critical medication such as chemotherapy for a sensitive animal?

Lili Duda, VMD, Section Editor of the OncoLink Veterinary Oncology Menu, responds:

There is very little information in the veterinary literature concerning transdermal medications in cats, even though their use is increasing because of the difficulty many owners have in administering oral medication to their pets.

Most drugs that are available in transdermal form are those that an animal requires on a daily basis to treat a chronic condition. Most of the problems with transdermal medications have to do with the very large uncertainty regarding the actual dosage that is being delivered to the patient. There is also some concern about individuals in close contact with the pets, as they in turn may be potentially exposed to medications that can be absorbed through the skin.

Most chemotherapy drugs have what is called a very narrow "therapeutic window", meaning that there is a small dose range over which the medication is effective. A little too low of a dose, and there might be no effect against the cancer. A little too high of a dose, and the side effects might be severe, debilitating, and even life-threatening. Therefore, even if a chemotherapeutic drug could be absorbed transdermally, this would not be an ideal route of administration. Furthermore, many chemotherapy drugs simply cannot be formulated for transdermal delivery.

There are many ancillary medications that are given to patients undergoing chemotherapy, such as anti-nausea medication, pain medications, and antibiotics. Very few are commercially available in transdermal form, although there are some that could theoretically be compounded into such a preparation by a specialized compounding pharmacy. This should be discussed with your veterinary oncologist on a case-by-case and drug-by-drug basis.

Most commonly, medications are formulated into a food-based liquid, paste, or chew to help increase palatability and ease of administration. There are several compounding pharmacies that specialize in veterinary products.