Factors that may predispose a patient to the development of type A adverse drug reactions
• Pharmacological factors: pharmaceutical, PK and PD drug interactions
• Pathological factors: renal, hepatic, cardiac and endocrine diseases
• Physiological factors: age-induced differences, species/breed PK and PD differences and drug disposition (ADME)
Avoiding adverse drug reactions
Type A (predictable) adverse drug reactions
• Obtain a proper diagnosis and treat according to recommended protocols, using least toxic drug(s) as appropriate.
• Evaluate patient before and during drug treatment with emphasis on target organ of toxicity (using appropriate diagnostic
tests) and remission of clinical signs.
• Minimize drug interactions.
• Educate client on the potential toxicities and associated clinical signs, and communicate frequently.
• Evaluate patient for remission of clinical signs, use the least amount of drug that will have therapeutic effect and
discontinue (or wean off slowly) as soon as drug is no longer needed.
• Use of compounds that help prevent metabolite damage to liver: N-acetylcysteine (intracellular form of glutathione),
ascorbic acid (oxygen radical scavenger) and S-adenosylmethionine (SAMe - contributor to a number of methylation reactions
in the body).
Type B (unpredictable) adverse drug reactions:
• Keep abreast on knowledge of potential occurrence of this type of adverse reaction.
• Frequent patient monitoring is important to avoid potential disastrous situations.
Diagnosing a drug-induced disease
• Ideally, the offending drug should be discontinued and side effects should be allowed to subside, then a challenge
with the same drug should show a similar undesirable pharmacological activity.
• Limitations include polypharmacy (i.e. difficult to determine which drug is the culprit) and ethical issues (especially
when dealing with anaphylactic reactions or severe toxicity).
Although drugs are important tools for treatment of several diseases affecting cats and dogs, they also invariably come with
potential predictable and unpredictable undesirable effects. It is not possible to list all possible drug-induced adverse
reactions for all drugs in all species. It is thus important to be vigilant and maintain a good veterinarian-client-patient
relationship in order to appropriately avoid these reactions or offer appropriate supportive treatment for our patients when
needed, and finally, to keep abreast on knowledge of potential occurrence of adverse drug reactions.
Boothe, D.M. 2001. Drug-Induced Diseases. Chapter 3 in Small Animal Clinical Pharmacology and Therapeutics. W.B. Saunders
Company. pp 41-59.
Cribb et al, 1996. Adverse reactions to sulphonamide and sulphonamide-trimethoprim antimicrobials: clinical syndromes and
pathogenesis. Adverse Drug React Toxicol Rev 15: 9-50.
Trepanier, L.A. 1999. Delayed hypersensitivity reactions to sulphonamides: syndromes, pathogenesis and management. Veterinary
Dermatology 10(3): 241-248.