Most Common Pet Drug Reactions
The most frequently reported adverse reactions in FDA veterinary drug safety reports, from 1,320,423 total records.
When a veterinarian or pet owner submits an adverse event report to the FDA, they describe what symptoms or reactions occurred. The most common reactions span a wide spectrum — from mild gastrointestinal symptoms like vomiting and diarrhea to serious neurological events like seizures and loss of coordination. The data below is based on keyword matching across all FDA CVM adverse event reports in our database.
Top Reported Reactions
Ranked by frequency across all species. Click Search to browse matching reports.
| # | Reaction | Reports | % of Total | Search |
|---|---|---|---|---|
| 1 | Vomiting | 210,445 | 15.9% | Search |
| 2 | Depression | 132,102 | 10% | Search |
| 3 | Lethargy | 90,713 | 6.9% | Search |
| 4 | Death | 81,504 | 6.2% | Search |
| 5 | Anorexia | 59,467 | 4.5% | Search |
| 6 | Seizure | 33,829 | 2.6% | Search |
| 7 | Ataxia | 33,010 | 2.5% | Search |
| 8 | Pruritus | 29,351 | 2.2% | Search |
| 9 | Weight Loss | 23,277 | 1.8% | Search |
| 10 | Alopecia | 17,937 | 1.4% | Search |
| 11 | Salivation | 16,232 | 1.2% | Search |
| 12 | Hypersalivation | 15,630 | 1.2% | Search |
| 13 | Weakness | 15,285 | 1.2% | Search |
| 14 | Polydipsia | 13,829 | 1% | Search |
| 15 | Trembling/Tremor | 12,845 | 1% | Search |
| 16 | Polyuria | 10,629 | 0.8% | Search |
| 17 | Inappetence | 8,869 | 0.7% | Search |
| 18 | Dermatitis | 8,506 | 0.6% | Search |
| 19 | Muscle Tremor | 2,270 | 0.2% | Search |
| 20 | Diarrhea | 0 | 0% | Search |
Counts are based on keyword matching in the reactions field. A single report may include multiple reactions; percentages are based on total reports in the database.
Common Veterinary Drug Reactions Explained
Vomiting and Gastrointestinal Reactions
Vomiting is the single most commonly reported adverse reaction in veterinary drug reports, particularly in dogs. It can occur with many drug classes including NSAIDs (like carprofen and meloxicam), antibiotics, and antiparasitic medications. Mild, transient vomiting shortly after drug administration is common and often not cause for discontinuation. Persistent, severe, or bloody vomiting warrants immediate veterinary evaluation. Diarrhea and anorexia frequently accompany GI reactions and are often grouped together as GI adverse effects.
Neurological Reactions: Seizures and Ataxia
Seizures and ataxia (loss of coordination/balance) are among the most alarming adverse reactions and are most commonly associated with isoxazoline-class flea and tick preventatives (such as fluralaner, afoxolaner, and sarolaner), as well as certain antiparasitic drugs. These reactions are more frequently seen in animals with pre-existing neurological conditions or breed-specific sensitivities (e.g., collies with the MDR1/ABCB1 mutation). Any seizure or significant neurological sign following drug administration should be treated as a veterinary emergency.
Depression and Lethargy
Behavioral changes including depression, lethargy, and reduced activity are among the most frequently reported non-specific adverse reactions. These can accompany almost any drug class and often represent mild systemic reactions. However, profound lethargy or unresponsiveness following drug administration should be evaluated promptly. In some cases, depression and lethargy are early signs of more serious systemic reactions including cardiovascular or hepatic effects.
Dermatological Reactions: Pruritus and Dermatitis
Skin reactions including itching (pruritus), rash, hives, and dermatitis can indicate hypersensitivity or allergic reactions to a drug. These are particularly common with topical treatments and some systemic medications. In severe cases, drug-induced hypersensitivity can progress to anaphylaxis — a medical emergency requiring immediate treatment with epinephrine. Any swelling of the face, difficulty breathing, or collapse following drug administration requires emergency veterinary care regardless of other symptoms.
Death and Euthanasia
Death is tragically one of the more commonly reported outcomes in serious adverse event reports. It is important to note that reported deaths are not always causally linked to the drug — the animal may have died from its underlying illness or from unrelated causes. The FDA uses a formal causality assessment methodology to evaluate whether a drug was likely, possibly, or unlikely to have contributed to an adverse outcome. Reports of death are still submitted even when causality is uncertain, as they contribute to pattern recognition and safety signal detection.
Frequently Asked Questions
Stop giving the medication and contact your veterinarian immediately. For severe reactions such as seizures, difficulty breathing, facial swelling, collapse, or extreme lethargy, go to an emergency veterinary clinic without delay. When you call, describe the drug name, dose, when it was given, and exactly what symptoms appeared and when. Do not give any additional medications without veterinary guidance. Keep the drug packaging and any remaining medication for the veterinarian to examine. After the emergency is addressed, consider reporting the reaction to the FDA through the FDA Safety Reporting Portal.
Isoxazoline products (including Bravecto, NexGard, Simparica, and Credelio) are FDA-approved and effective for most pets, but the FDA added a warning label in 2018 requiring disclosure of potential neurological side effects. A small percentage of animals — particularly those with pre-existing seizure disorders or the MDR1 genetic mutation (common in collies, Australian shepherds, and other herding breeds) — may experience muscle tremors, ataxia, or seizures. For the vast majority of healthy dogs and cats, these products are considered safe. Discuss the risks with your veterinarian, especially for breeds with known neurological sensitivities.
Not necessarily. Adverse event reports describe a temporal association — the animal received a drug, and then experienced a reaction — but they do not prove causation. The FDA uses a structured causality assessment process to evaluate whether a drug was likely responsible for the reported reaction. Many reports involve animals with pre-existing conditions, concurrent medications, or environmental factors that could independently explain the symptoms. This is why adverse event data must be interpreted carefully: high report volumes may reflect high drug usage rather than high risk.
Based on FDA CVM adverse event data, the drug classes with the most reports in dogs include: isoxazoline-class ectoparasiticides (flea/tick preventatives), NSAIDs used for pain and inflammation, macrocyclic lactones (heartworm preventatives), and certain antibiotics. NSAIDs in particular carry a well-documented risk of gastrointestinal, hepatic, and renal adverse effects with long-term use. Flea/tick products generate high report volumes partly because of widespread use in the pet population. Reporting volumes should always be considered in the context of total prescriptions dispensed.
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