Human OTC Products for Veterinary Patients

Introduction

Many human over-the-counter (OTC) drug products are potentially toxic to dogs and/or cats, even at low doses, owing to species differences in drug disposition.



Analgesics

In human beings (and dogs), paracetamol (acetaminophen) is metabolized to nontoxic metabolites via glucuronidation and sulfation pathways with the conjugates. However, cats lack the glucuronyl transferase enzymes required to metabolize the drug and result in the production of toxic NAPQI as the main metabolite. Plus, relative to dogs and humans, cats are deficient in glutathione, which is important for detoxifying NAPQI. Within 60 minutes of a toxic dose of paracetamol, NAPQI will oxidize susceptible amino acid residues of feline haemoglobin, converting it rapidly to methaemoglobin, resulting in tissue hypoxia.

Paracetamol Poisoning in Mouse

Human NSAID drug products should not be recommended without first consulting with a veterinarian. Veterinary NSAIDs should be used instead.

  • The relative COX2 selectivity for each NSAID can vary greatly between species. It is not predictable based on drug class or structure.
    • Meloxicam is COX2 selective in humans and dogs, but not in cats.
    • Piroxicam shows COX2 selectivity in dogs, but not humans.
  • NSAID metabolism in cats and dogs can vary widely relative to that of humans, depending on the particular drug. In general, cats are thought to metabolize NSAIDs more slowly than dogs or humans, necessitating longer dosing intervals.
  • NSAIDs can cause GI ulceration, hepatopathies and impaired renal function.



Allergy, Cough, Cold and Flu

OCT combination products (e.g. with NSAIDs, acetaminophen, decongestant and cough suppressants) should be avoided.

  • The dose ratios were designed to meet the general pharmacokinetic and pharmacodynamic needs of human patients.
  • The therapeutic window for many of the individual drugs in these combination products is very narrow (or non-existent) for dogs, cats or other veterinary species.
  • There are many FDA-approved veterinary drugs that are more effective and safer.

Several OTC antihistamines can be used therapeutically in both human and veterinary patients.

  • First-generation antihistamines (e.g. chlorpheniramine, diphenhydramine) have been used more frequently than second-generation antihistamines (e.g. cetirizine, loratadine).
  • The sedation and drowsiness due to first-generation antihistamines is generally considered an advantage, particularly for pruritic pets.

Decongestants (pseudoephedrine and phenylephrine) have a narrow therapeutic index in dogs and cats.

Dextromethorphan (cough suppressant) is not considered efficacious in veterinary species.

  • Consider butorphanol to relieve cough in dogs.
  • No antitussive available for cats.



Gastrointestinal

The histamine-2 receptor antagonists and proton pump inhibitors are more effective in combating gastric acid than antacids (such as aluminium hydroxide and calcium carbonate).

  • Calcium carbonate is more often prescribed as phosphate binders for dogs or cats with hyperphosphatemia secondary to renal failure.
  • Famotidine is the H2 receptor antagonist least likely to cause an adverse event because it does not have prokinetic actions (ranitidine and nizatidine are prokinetics), nor does it inhibit cytochrome P450 enzymes (cimetidine does).
  • Pet owners should be discouraged from administering PPIs to their pets unless specifically prescribed by a veterinarian because of the potential for secondary complications.

Antidiarrheal drugs such as loperamide are seldom needed or administered. Loperamide can pose high risk of neurological toxicity in dogs or cats with polymorphisms of the MDR1 (ABCB1) gene.

Osmotic laxative drugs (e.g. lactulose) is most likely to be prescribed for constipation in cats and dogs. However, cats and some dogs do not like the taste of lactulose.

  • Commercially available enemas formulated for people can be deadly in cats and small dogs. They can create severe electrolyte imbalances, dehydration and hypotension.



Dermatologic

Short-acting topical corticosteroids (e.g. betamethasone) are useful for controlling focal areas of pruritus but should not be used chronically.

  • Some pets will show signs of iatrogenic Cushing's disease after topical corticosteroid use, as they can be absorbed systemically.
  • Topical corticosteroids in cats are difficult to restrict to topical use, as “what’s on the cat, is in the cat” (cats are notoriously fastidious in their grooming and licking and may ingest topical products)

Topical antifungal agents (e.g. miconazole, clotrimazole, terbinafine) are used for dermatophytes and Malassezia skin infections in dogs, cats, horses and other species.



Ophthalmic

The use of artificial tears or natural tear replacement solutions is similar to that in human patients.

Because most dogs and all cats weigh substantially less than adult humans, Systemic absorption of drugs administered topically to the eye, particularly in the form of drops, can result in systemic adverse drug effects.

Drops versus ointment: Drugs administered topically to the eye can also reach the nasopharynx by way of the nasolacrimal system, where the drug can stimulate chemoreceptors (taste buds). Cats are particularly sensitive to unpleasant tastes and will salivate profusely if their taste buds are confronted with a noxious taste. This may cause anorexia and treatment aversion in a species that is notoriously difficult to treat to begin with. For this reason, ophthalmic ointments are often preferred to ophthalmic drops for cats. Atropine appears to be particularly distasteful to cats.



Xylitol

The artificial sweetener, xylitol, used in human OTC and prescription drug products, is highly toxic to dogs.

  • In canines, xylitol causes an immediate release of insulin from the pancreas, which can lead to profound and potentially life-threatening hypoglycemia.



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