Transdermal medications, as we learned previously, are meant to absorb into and distribute themselves throughout the body. Creams, sprays, and patches containing steroid hormones such as estrogen or testosterone absorb very easily through the skin, because of their chemical structure. That’s why it’s so easy for a pet to be poisoned by simply touching the treated area of skin. Most other drugs, however, need to be manipulated in some way to make them enter the body through intact skin. One way to do that is to load up the cream, or the patch, or whatever vehicle is being used for treatment, with enormous quantities of drug to help force the active ingredient across the skin barrier.
One example is the fentanyl patch. You know from the daily news that fentanyl is extremely dangerous when taken in the wrong quantity. On the other hand, it is a miraculous pain killer when used in precisely calculated amounts for patients with cancer, burn wounds, or a recently reconstructed skeleton. One of the best ways to suppress ongoing pain like that is with a fentanyl patch, which delivers a steady trickle of drug into the body of the patient. The patch provides enough drug for 72 hours, but even the discarded “sticker” can contain as much as 84% of the original fentanyl. One bright spot—the fentanyl molecule loses much of its power on entering the stomach, so pets that swallow the device whole could actually survive their ordeal without much fuss. However, chewing and licking the spent patch delivers an extremely high drug concentration into the blood stream and is thus much more likely to kill.
Another especially toxic transdermal, used to help cigarette smokers quit their habit, can easily be obtained without a prescription, and it’s actually more consistently lethal than a pain patch. Nicotine patches, like fentanyl patches, release only a portion of their content into the intended patient. The full payload, therefore, of a single “step one” 21-mg patch is over 100 mg of nicotine—easily sufficient to kill a 20-pound dog if ingested. Like fentanyl patches, a large amount of drug is left in the device even after it has finished its useful life on human skin, but—unlike fentanyl—nicotine is easily absorbed from the stomach and intestine. Symptom onset is both rapid and dramatic, manifesting as salivation, vomiting, tremors, dilated pupils, and seizures. Death, of course. Fentanyl has a specific antidote, while nicotine does not.
As you can see, patch medication needs to be treated with a great deal of respect. It’s tempting to view the spent product as “disposable” or “trash”, although a more fitting descriptor might be “lethal weapon”. There are several other transdermal patches on the market, and they all behave similarly (i.e., very high drug content if taken orally). The safest way to handle such devices is to keep them well out of the reach of pets and children, and to ultimately dispose of them through a pharmaceutical take-back program. In our next installment, you’ll learn about the final four—the most dangerous compounds on our list.
Dr. M.S. Regan