Toxic Topicals Part One

In preparation for our short series on topical pharmaceuticals, let’s first understand some basics about this kind of medicine. The word “topical”, in this context, refers to a drug that is applied to the outside of the body. These are meant to act on the immediate area, like when you apply bacitracin to a minor scrape. There is another kind of medicine that is applied to the skin but is then absorbed deep into the body; the correct term for that is “transdermal”, and you will soon be reading about both. Another point to understand is that dogs and cats are much more injudicious than (most) humans about what they put into their mouths. They are also (many of them, at least) much more flexible than a human being. This explains why your pet might be eating a topical medication straight from the tube, or else ingesting it off the skin (their own, or the skin of their beloved owner). Even though it doesn’t immediately occur to most pet owners, tubes of skin cream or foam can pose a serious threat to their animal companion through seemingly innocuous contact.

Here’s an example: it was only relatively recently discovered that hormone applications appropriately prescribed for a human can have damaging effects on the household pets, even though they are applied exactly according to the instructions. Made from substances like estrogen, progesterone, or testosterone, these medicines are transdermal, which means they can easily enter the body and enact systemic changes for anyone touching them. Pets that come in contact can experience severe sexual effects such as dramatic enlargement of the vaginal or prostate tissue, recurrence of heat cycles, swelling of the uterus or its remnants, and even failure to produce blood cells. Those things are generally reversible, and they are most likely to occur after repeated exposure to the medication. However, without the knowledge that your medicine could harm your pet, you wouldn’t think to bring this to the veterinarian’s attention. It could take extensive diagnostics and even an endocrine specialist to get the details we need to change your household habits and save your pet’s life.

Another medicine that can harm your pet is zinc oxide, a gentle, soothing skin cream and wonder drug for painful diaper rash. It has its place in veterinary medicine when severe inflammation arises in an area that gets too much moisture and not enough air (usually also the “diaper area”, in patients with excessive skin folds or severe incontinence). The zinc portion is poisonous, but only in large amounts. An entire tube eaten by your pet will nearly always stimulate him to vomit profusely, which will expel most of the toxin from the body. However, ingesting a small amount daily by licking medicated skin can be substantially more dangerous as it accumulates to a high concentration inside your pet. Zinc oxide is a topical medicine, so it is not absorbed through the skin. The problem arises when we apply this easily-obtained, innocent-seeming balm for long periods of time without the advice of a veterinarian.

In the following installments, we’ll explore increasingly dangerous medications and how they may go about threatening your pet. You may be surprised to discover that one of these is lurking in your own home.

Dr. M.S. Regan