Adverse Reactions to Flea, Tick and Worming Products

This guide is written by Dr. Kimberley Chainey BVSc, one of our qualified in-house veterinarians.

We all know the importance of protecting our pets from parasites such as fleas, paralysis ticks, heartworm and intestinal worms. Most pet parents will have also applied a topical product, collar or administered an oral chew to their pet at some stage.

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.

1. Minimising the chance of an adverse reaction


It is not always possible to predict which animals will react to a given product until a reaction has occurred. For example, the first time my sister found out she was allergic to Penicillin was when she was prescribed this particular antibiotic. Despite this, there are ways to minimise the chance of an adverse reaction occurring.

  • For oral preventatives, such as Comfortis, Nexgard Spectra, Bravecto, 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 fleas and paralysis 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.

  • Some oral preventatives contain flavouring made from pork and beef-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, Comfortis 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 Isoxaline as the active ingredient, including Nexgard, Nexgard Spectra, Simparica, Bravecto Spot On and Bravecto Chewables.



2. How to Identify an Adverse Reaction to a Topical Product

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.

3. Managing Adverse Reactions to Topical Products

If your pet appears agitated, hyperactive, quiet, subdued or reluctant to remove after the administration of a topical product:

  • Bath off the product with a mild dish soap. Do not use pet shampoo as it is too mild to remove all of the product.

  • Report the adverse reaction to the manufacturer. Product labels and product websites will have a contact number to report directly to the registration holder. By law, manufacturers are required to send monthly reports of adverse reactions to the appropriate regulating authority including the APVMA (Australian Pesticides and Veterinary Medicines Authority).

  • Keep any of the remaining product and packaging in a safe place in case a sample is required by the registration holder.

  • Do not use the same product in the future. Try a product with a different active ingredient or consider a different formulation (e.g. oral). Observe your pet closely for at least an hour after application or administration of future preventatives.

If your pet develops red and irritated skin, blisters or ulceration:

  • Bath off the product with a mild dish soap. Do not use pet shampoo as it is too mild to remove all of the product.

  • Please note that often the onset of skin irritation is delayed for several hours with the full extent not apparent until 12-24 hours after application.

  • Seek veterinary advice for further management.

  • Report the adverse reaction to the manufacturer. Product labels and product websites will have a contact number to report directly to the registration holder. By law, manufacturers are required to send monthly reports of adverse reactions to the appropriate regulating authority including the APVMA (Australian Pesticides and Veterinary Medicines Authority).

  • Keep any of the remaining product and packaging in a safe place in case a sample is required by the registration holder.

  • Do not use the same product in the future. Speak with your veterinarian about the best course of action for administration of further parasite prevention products.

4. How to Identify an Adverse Reaction to an Oral Product

The most commonly observed adverse reactions to oral preventatives include vomiting, lethargy, a reduced appetite, diarrhoea and pruritus (itching). Preventatives belonging to the Isoxaline 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 Isoxaline family such as Nexgard, Bravecto and Simparica. If seizures develop after administration, your veterinarian will likely advise to avoid these products in the future.

5. Managing Adverse Reactions to Oral Products

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:

  • Contact your regular veterinarian to determine whether or not your pet needs to be redosed.

  • In general, if vomiting occurs within 1-3 hours, your veterinarian may recommend redosing 24 hours later. They may also recommend administering with food.

  • If vomiting occurred after 4 hours, the product has most likely already been absorbed and redosing may not be required.

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.