Recognise the signs of a reaction
Mild reactions can include redness, itching, or irritation at a topical application site, or mild digestive upset after an oral chew. More serious reactions can include vomiting, diarrhoea, excessive drooling, tremors, wobbliness or loss of coordination, seizures, facial swelling, hives, or difficulty breathing. Reactions can appear within minutes of application in some cases, or develop over several hours, so it's worth watching your dog reasonably closely for the rest of the day after any new treatment. This matters especially the first time you use a particular product, since you don't yet know how your individual dog will respond to it.
What to do for a mild reaction
If your dog shows mild skin irritation at a topical application site, you can generally wash the area with mild soap and water once enough time has passed per the label (check first, since washing too soon can reduce the product's effectiveness), and monitor. For mild digestive upset after an oral product, withholding food briefly and offering water is often reasonable, but call your vet if it doesn't settle within a few hours or if your dog seems unwell rather than just mildly off.
What to do for a serious reaction
Facial swelling, hives, difficulty breathing, collapse, seizures, or significant tremors are emergencies. Get your dog to a vet or emergency animal hospital immediately, don't wait to see if it passes, and don't give another medication to try to counteract it without veterinary guidance. If you can, bring the product packaging with you so the vet can see exactly what was used and when.
Report it, don't just switch products quietly
Tell your vet about any reaction, even a mild one, so it's recorded in your dog's history and can be avoided in future. Many manufacturers also have an adverse event reporting process, and your vet can advise on whether to use it. This matters beyond your own dog, reaction reports help identify patterns with specific products or ingredients.
Choosing a different product afterwards
Don't assume every flea and tick product carries the same risk just because one caused a reaction; different active ingredients affect individual dogs differently. Your vet can help identify what likely caused the reaction and recommend an alternative product or approach less likely to trigger the same response, taking into account your dog's breed, age, health history, and any other medications they're on. Keeping a simple note of which products your dog has reacted to in the past, and sharing it at every vet visit, helps avoid repeating the same reaction with a differently branded product that contains the same ingredient, even years later with a different vet clinic.