From the Archives, Summer 2018

Similar but Different Part Two: Human and Veterinary Patients

Vet Human Drugs
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By: Heather Kidston,

In the spring edition of Pharmacy Connection, some differences in veterinary and human marketed drugs were presented to raise awareness of the challenges they may present in a retail pharmacy setting. The purpose of this article is to present a few real-life pharmacy situations that highlight veterinary pharmacology and toxicology.

Consider how the following differences would affect drug dosing:

  • Dogs cannot acetylate. What drugs would this affect?
  • Cats cannot glucuronidate. What drugs would this affect?
  • The GI transit time is 6-8 hours in dogs vs 20-30 hours in humans1
  • The bioavailability of azithromycin is 97% in dogs, 37% in humans2
  • The bioavailability of clonazepam is 33% in dogs, 90% in humans2


JS presents a prescription for Synthroid 0.3mg BID for his dog, Ghost. The pharmacist is familiar with the veterinary prescribing short form “SID” instead of “OD” used for once daily dosing, and reads the prescription as Synthroid 0.3 mg SID. The CPS states that “few patients require doses greater than 0.2 mg/day.” Using professional judgement and the information at hand, the Rx is dispensed as Synthroid 0.3 mg once daily.

Two veterinary pharmacy facts are presented in this case. The first is “SID” as the Rx abbreviation for once daily in veterinary medicine. Several reported dispensing errors involve SID and BID being confused.

The second is the high dosing of levothyroxine in dogs when comparing it to human dosing. This difference has also resulted in dispensing errors. The half-life of levothyroxine in canines is only 10 – 16 hours, compared to ~7 days in humans.4 The CPS suggests an approximate initial adult dose of 1.7 µg/kg/day administered once daily. Canine levothyroxine dosing can range from 20 µg/kg/day to 20 µg/kg BID.4 For context, based on average weight, that would be 200 µg for a beagle, 600 µg for a golden retriever, and 800 to 1000 µg for a great dane.


NL presents a prescription for mirtazapine OD 30 mg for her dog, Larry. When the pharmacist inquires, NL indicates that it has been prescribed as an appetite stimulant. The pharmacist knows that xylitol is toxic to dogs. Before proceeding to fill the prescription, the pharmacist ensures that xylitol is not an active ingredient in the generic brand of mirtazapine OD currently stocked.

Xylitol can be toxic to dogs in doses as low as 0.1 – 0.5 mg/kg.3 For a 2 kg Chihuahua that works out to 200 mg – 1 g of xylitol. Often the quantity of xylitol is not stated on product labels. One of the higher xylitol containing products is “PUR” brand mints and gum, which contain about 1 gm of xylitol per piece. Watch for xylitol in dry mouth treatments, liquid dosage forms, nasal sprays, chewable tablets, nicotine gums or orally dissolving formulations. Avoidance of toxic non-medicinal ingredients such as xylitol, propylene glycol or benzyl alcohol is also a reason for a veterinarian to request a certain brand of a drug with “no substitution.”


SK approaches the pharmacist to ask about the best option for pain relief for her cat, Peaches, who got her paw caught in the patio door. She is considering acetaminophen, ibuprofen, ASA, or naproxen in the OTC pain relief section. Which of the following are correct?

A) SK can give Peaches ibuprofen infant drops (40 mg/mL) according to the dosing chart on the side of the box (based on Peaches’ weight)
B) SK can give ibuprofen infant drops (40 mg/mL) but should call her veterinarian to check the dose in mg/kg, because it is different in every species
C) Although the 1/2 life of ASA in humans is 1.5-4 hours, in cats it ranges from 38-45 hours
D) Acetaminophen is contraindicated in cats at any dosage
E) Naproxen is highly toxic to dogs, cats, and ferrets
F) Since what is considered safe for humans is not always safe in cats, SK should consult her veterinarian before medicating Peaches.

C, D, E and F are all correct.

Naproxen, acetaminophen and ibuprofen are all toxic to cats.3 ASA is rarely used in cats because of their impaired ability to metabolize the drug. If prescribed, it would require close monitoring and would be dosed q48-72h.

The reason “F” is correct, and what pharmacists likely do not know, is that the use of a human OTC in any other species is classified as extra label drug use. Veterinary oversight is expected if a drug is not Health Canada approved for use in the species being treated (ie an approved dose for cats on the package or insert material). Although you cannot prevent a client from purchasing an OTC for their pet, if asked, recommend that they consult their veterinarian.

The use of a human OTC in any other species is classified as extra label drug use. Veterinary oversight is expected if a drug is not Health Canada approved for use in the species being treated (ie an approved dose for cats on the package or insert material).


VF is purchasing a box of Benadryl Total. When asked if he has any questions about the medication, he explains that he is on his way home from his veterinarian, and was instructed to purchase it for his dog. He is surprised and thankful when the pharmacist intervenes to tell him that this product contains pseudoephedrine and acetaminophen in addition to diphenhydramine, which are both toxic to dogs.3 The prescription from his veterinarian says “Benadryl tablets.” VF had inadvertently chosen the wrong Benadryl product in the allergy section.

Pseudoephedrine is toxic to both dogs and cats. In dogs, 10-12 mg/kg can be lethal. Acetaminophen is toxic to cats, and although it can be used in dogs, doses above 100 mg/kg can be toxic.3 This common OTC self-selection mistake can be especially dangerous to veterinary patients.

When providing care for animal patients, pharmacists must be aware that veterinary pharmacology does not always correlate with human pharmacology. This can make counselling challenging and could lead to contradictory recommendations and confusion. Awareness and communication are the keys to quality continuity of care for animal patients and their caregivers.

  1. Martinez et al, Journal of Pharmaceutical Sciences 98 (2009)844-60
  2. H Musther et al, European Journal of Pharmaceutical Sciences 57 (2014) 280-291
  3. Brutlag, A. Drug Store Toxins in Small Animals, Plumb’s Veterinary Drug Handbook, 2015
  4. Plumb, DC. Product Monographs, Plumb’s Veterinary Drug Handbook, 2015

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