Last Updated: 29/03/2025
Learn how to minimise ricks in adverse reactions to flea and worming medication in dogs and cats
Author: Dr Teagan Lever BVSc (Hons)
Reading Time: 27 minutes - medium read
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Parasite control is a crucial part of responsible pet ownership, protecting our furry friends from a range of harmful pests. While these treatments are generally safe and effective, it's important to be aware that rarely adverse reactions can occur. Rest assured, these reactions are typically mild and temporary, and with prompt attention, they can be easily managed. By understanding the potential side effects and knowing what to do in case of a reaction, you can ensure your pet's continued health and well-being.
When used according to the label directions, these products are generally well tolerated by most pets. Remember, adverse reactions do not mean that a drug is unsafe and the vast majority of animals will not experience any side effects.
It is not always possible to predict which animals will react to a given product until a reaction has occurred. Despite this, there are ways to minimise the chance of an adverse reaction occurring.
For oral preventatives such as Nexgard Spectra, Bravecto chews, Simparica and so on, this means purchasing the correct size for your dog or cat and not splitting tablets for cost saving benefits. Splitting tablets that are not scored with a clear graduated line down the middle, no matter how careful you are, increases the risk of over- and under-dosng.
For spray formulations, follow the directions carefully to ensure the correct volume is delivered and at the correct dosing interval. For example: For a 5kg dog with a short coat, 30 pumps are required of Frontline Spray to achieve sufficient protection and must be redosed again on a 3 week basis to protect against ticks.
Check the packaging and safety information to ensure the product is suitable for your pet's age and weight. Many parasite prevention products cannot be used until your pet has reached a minimum age, commonly 6-8 weeks of age. Many oral flea and tick preventatives also cannot be used until your pet reaches a minimum body weight of 2kg.Note: Simparica is one exception and is suitable to give to puppies from 1.3kg body weight from 8 weeks of age.
Many products have not been tested in breeding, pregnant and lactating animals. For these animals, we recommend consulting your veterinarian for further advice, and using a product with proven safety such as Bravecto, Frontline Plus or Advantix (for dogs only).
Some oral preventatives contain flavouring made from pork, beef or soy-based protein. If your pet has an underlying food intolerance to a particular protein, double check that the product is free from the offending protein. For example, Credelio Plus is formulated with pork flavouring while Nexgard Spectra is a vegetable-based chew using soy proteins.
Some oral products recommend dosing with food to improve efficacy and reduce the risk of an adverse reaction such as vomiting. Check the packaging insert prior to administration.
Never use a product formulated for dogs on cats. Cats are unable to metabolise certain compounds and administration can result in systemic toxicosis which can be life-threatening. The classic example is Advantix which must never be administered to cats.
If your pet has an underlying health condition, always discuss with your veterinarian prior to administration. This is particularly important in cats and dogs with underlying neurological disorders as some products emphasize using caution with a history of epilepsy or seizure disorders. This applies to products which contain an Isoxazoline as the active ingredient, including Nexgard, Nexgard Spectra, Simparica, Simparica Trio, Bravecto and Bravecto Plus Spot On and Bravecto Chewables.
Want to know more? Read Can you split parasite prevention tablets in half? for more information.
Some animals may have an adverse local reaction to the ingredient or ingredients within a topical product. In some cases, they may also react to other topical products which contain a completely different active ingredient, suggesting that these animals may be sensitive to some of the inactive ingredients within the product. Unfortunately, it is not possible to predict which individuals will react to a given product until a reaction has occurred.
In all cases, these are considered a local hypersensitivity reaction and do not reflect a systemic toxicosis. The signs can vary from mild tingling sensations, to chemical burns, vomiting, hair loss, lethargy and decreased appetite. In cats and dogs, mild tingling sensations may manifest as agitation and hyperactivity as they try to get away from the sensation, while others may be quiet, subdued and reluctant to move. The severity of the skin irritation can vary from no obvious changes, to blisters, and even ulceration at the site of application.
The most commonly observed adverse reactions to oral preventatives include vomiting, lethargy, a reduced appetite, diarrhoea and pruritus (itching). Preventatives belonging to the Isoxazoline family also rarely report seizures. If your dog or cat has a history of seizures or neurological signs, your veterinarian should be consulted prior to administration of products belonging to the Isoxazoline family such as Nexgard, Nexgard Spectra, Bravecto, Bravecto Plus, Credelio, Credelio Plus, Simparica and Simparica Trio. If seizures develop after administration, your veterinarian will likely advise to avoid these products in the future.
Most adverse reactions such as lethargy and a reduced appetite are self-limiting and of a short duration. If vomiting occurs after administration, your veterinarian may recommend redosing depending on the time interval. If your pet does vomit after administration:
Importantly, if your pet does experience an adverse reaction to a topical or spot on preventative, report it to your veterinarian and the manufacturer so an appropriate investigation can be carried out. Avoid the media scrutiny of certain products which ultimately only incites fear and distress among pet parents looking to do the best by their animals. Remember, while some of these negative reviews and reports will be true, they represent the small percentage of adverse reactions we expect with any form of treatment. In the vast majority of these cases, severe adverse reactions such as liver failure, kidney failure and death are completely unrelated - keep in mind that correlation does not imply causation. Appropriate reporting will ensure these products are closely monitored and at the end of the day, is our best chance of protecting our pets.
If your pet appears agitated, hyperactive, quiet, subdued or reluctant to remove after the administration of a topical product:
If your pet appears agitated, hyperactive, quiet, subdued or reluctant to remove after the administration of a topical product:
Our experts continually monitor the health and wellness space and we update our articles when new information becomes available.
Contributing Author, BVSc
Kimberley graduated from James Cook University in 2012 and initially worked in a mixed animal practice on the Atherton Tablelands for two years. Her interest in small animal medicine led her to undertake a rotating internship at Veterinary Specialist Services in Brisbane. Since then, she has worked in small animal clinics throughout south east QLD and NSW! She is passionate about providing the best quality of care to all animals that walk through the door
