Veterinarians’ Decisions In Pharmaceutical Dosing

 “D” wrote to yesterday to ask for the canine dosage of a commonly-used human preparation, diphenhydramine, which goes by the trade name Benadryl.

I politely declined to advise a dosage for several reasons, which I shared with “D.” I will expand on those reasons today.

Chihuahuas have a high body-surface-area to weight ratio.
Chihuahuas have a low weight to body-surface-area ratio.

First, while I am licensed to practice veterinary medicine in three states, I hold an active license in only one state. Giving medical advice outside this state would be a violation of “D’s” state’s veterinary medical practice act.

Second, “D,” his pet and I lack a doctor-client-patient relationship. This is a medicolegal association, outside of which I am legally prevented from giving specific advice. “How, then,” you might ask, “are you able to answer readers’ questions?” The answer is that I do not give specific patient advice, I render only my opinion and I send the pet owner back to his regular veterinarian for care.

Third, I’m not a fan of Benadryl for dogs. I find that it has very little usefulness in veterinary medicine, despite its excellence at blocking certain allergic reactions in people.

Fourth, and this goes back to a concept hinted at in the post, Dosages May Vary. “D’s” dog weighs 225 pounds. He questioned the dosage he found online, as do I in the post 64% Of Medical Information On The Web Is Wrong Or Outdated.

Lacy, like all Great Danes, has a high weight to body-surface area ratio.
Lacy, like all Great Danes, has a high weight to body-surface area ratio.

As most of you know, we doctors attend years of school on many, many scientific subjects prior to graduating with a degree. We are then tested extensively at the national and state levels before we are granted licenses. (One is not allowed to practice with a degree alone.) One legal requirement for renewing the license is proof of attendance for the necessary number of hours of continuing education.

Put another way, the practice of veterinary medicine is more than looking up a dosage in a book and administering a medication.

One of the principles learned in the study of pharmacology, “the science of drugs, especially with relation to their therapeutic value” (Webster’s New Collegiate Dictionary), is that while some drugs are dosed on patient body weight, others are dosed on body surface area. Common sense tells us that the Chihuahua pictured above has both a lower body weight and smaller body surface area than the adjacent Great Dane.

However, calculus reveals to us that the Chihuahua’s weight-to-body-surface-area ratio is much higher than the same ratio applied to the Great Dane.

Therefore, certain medications’ dosages might seem quite high for the Chihuahua, while the same medication dosed for the Great Dane might seem like too little. Such intricacies as medication dosages should be reserved for your pet’s doctor.

Fifth, and perhaps most important, I asked “D,” why his dog needed Benadryl, or, put differently, what makes him think his dog needs Benadryl?

The answer to that question most likely lies in “D’s” dog’s veterinarian’s office.

See you Monday, Dr. Randolph.

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