An adverse drug reaction is a term used to describe an undesired response to a medicinal product. Any cat can develop a reaction to any drug. Symptoms (especially severe ones) will usually appear within a few hours but can be up to 2-3 days later. Adverse drug reactions can vary from very mild to severe (including death). Most drug reactions only cause mild symptoms; severe reactions are very uncommon.
Causes
The most common reported reactions are those from:
Vaccinations
Antimicrobial medications, such as antibiotics
Non steroidal anti-inflammatory medications (NSAIDs)
Parasite preventatives
Anaesthetic agents
These are the most commonly used medications in veterinary medicine, so it’s likely that this is why we see more reactions reported, rather than these being more risky.
Risk
Several factors may increase the risk of adverse reactions:
Both very young and elderly cats may be more likely to have reactions to medications.
Cats with liver, kidney or heart disease.
Cats with allergies (atopy).
Cats with the MDR1 gene mutation have a higher risk of reactions to certain medications, such as Ivermectin. This is breed-specific in dogs but can occur in any breed of cat.
Cats that have had a previous reaction to a medication are more likely to have a repeat reaction to the same medication. These further reactions can be more severe.
Symptoms
The most common symptoms of drug reactions include:
Vomiting and diarrhoea
Lethargy
Reduced or increased appetite
Increased thirst
Skin reactions: itching, redness, hives and hair loss
Drooling
Sedation
Severe allergic reactions are rare but life-threatening. Symptoms include:
Difficulty breathing
Red or blue gums
Collapse
Diagnosis
Adverse drug reactions are difficult to diagnose, as the symptoms tend to be quite vague and can have many different causes.
Treatment
Treatment depends on the severity of the reaction. Typically, it involves removing any remaining drug from the body and providing symptomatic relief.
This may include:
Emergency treatment for anaphylactic reactions
Anti-seizure medication
Antihistamines and/or corticosteroids
Activated charcoal: to reduce further absorption of the drug
Induced vomiting (if caught early)
Antidotes: if available, though these are not common in vet medicine
How to help at home for mild symptoms
Feeding small meals of bland food can help with tummy upsets
What to expect
Most mild drug reactions should resolve within 24-48 hours. Long-term problems are not to be expected.
Less common problems include:
Transient problems in the liver and kidneys. These often improve once the medication is stopped.
Immune-mediated disease, such as IMHA.
Stomach ulcers caused by NSAIDs and steroids.
Lumps caused by injection reactions.
Neurological signs, such as incoordination and seizures due to flea or tick preventatives.
Prevention
Always stick to the dose that your vet has prescribed. If the dose is not clear, double-check before using it.
If you miss a dose or overdose by accident, always check with your vet before giving any further medication.
Never give your cat human medications, these can be toxic to them.
For cats on long-term medication, attend regular check-ups with your vet. Routine blood and urine tests are advised for monitoring certain medications.