An idiosyncratic adverse drug reaction is a clinical drug reaction, not a diagnosis. It refers to an unpredictable, abnormal response to a medication given at an appropriate dose, occurring in a small subset of dogs or cats.

Unlike overdose or dose-dependent toxicity, idiosyncratic reactions do not occur in most patients, do not reliably worsen with higher doses in the general population, and are not related to the intended drug effect. They are often delayed and may appear days to weeks after starting a medication, or much faster after re-exposure.

These reactions are believed to involve reactive drug metabolites and immune system activation, which can damage the liver, skin, blood cells, or multiple organ systems, even when the medication itself was used correctly.

Locally and globally, idiosyncratic drug reactions remain one of the most challenging causes of sudden illness in medicated pets because they cannot be predicted in advance and may initially look unrelated to the drug.

This condition is commonly searched by pet owners using phrases such as unexpected drug reaction in dogs, dog got sick after starting medication, cat reaction to medication, delayed drug reaction pets, drug allergy vs toxicity dog, side effects at normal dose dog, pet liver failure from medication, dog skin reaction to meds, cat jaundice after medication, drug reaction weeks later dog, carprofen liver reaction, methimazole reaction cat, oral diazepam toxicity cat, phenobarbital blood disorder dog, sulfa drug reaction dog, drug-induced anemia dog, drug-induced skin disease dog, medication reaction not overdose, idiosyncratic drug reaction pets, and pet adverse drug event, reflecting how frequently these reactions occur despite correct dosing.