How Do Flea Collars Work?

By Hayley Bruning
Excessive scratching, red bumps, hair loss—these are symptoms no one wants their pup or cat to experience. Yet these are all common signs your pet is carrying fleas.
Along with causing your pet discomfort, fleas carry and spread other parasites and infections, such as tapeworms and flea allergies. In order to get ahead of a flea problem—especially as flea season approaches—pet parents can take preventative measures. One option is getting your dog or cat a flea collar.
Flea collars vary by type and usage, and learning about some common products can help you choose the most effective one for your pet.

Image via PetMeds
What Are Flea Collars, and How Do They Work?
Flea collars contain medicine that, upon release, either repel or kill fleas on its host. During the course of treatment, the medication is absorbed through the oil glands of a pet’s skin and fur. It is then spread throughout the pet’s body, reaching the target areas where fleas are present.
Flea collars are designed to be worn for around four to eight months, depending on the collar’s specifications. Most products specify an age range on the packaging, so you’ll know whether it’s safe for puppies or kittens.
No two flea collars are exactly likely, and in fact, there are two main types of flea collars pet parents should know.
Repellent Collars
Repellent collars release active ingredients that repel fleas before they bite or even land on a pet. Many repellent flea collars on the market contain a blend of essential oils, along with inactive ingredients like nylon and zinc stearate. Some may also use insecticide.
Treatment Collars
Treatment collars typically contain insecticides and are designed to kill fleas on contact. Some common insecticides you’ll find in treatment collars include imidacloprid and flumethrin. Several products may also include insect growth regulators, like methoprene.
Are Flea Collars Effective?
Flea collars can be just as effective at treating and preventing fleas as oral medications and topical droplets. That said, pets can have various reactions to new medications. Consult your veterinarian before trying any new product; they know your pet’s medical history and can provide personalized recommendations.
Flea collars are most effective when fitted properly on a dog or cat. If a collar is too loose, the full dose of medication may not reach the pet’s skin. On the other hand, an overly tight collar raises the risk of strangulation. Check that your cat or dog is comfortable and make sure you can fit two fingers underneath your pet’s collar.
Are Flea Collars Safe for Dogs and Cats?
There are several safety considerations to keep in mind when choosing a flea collar to try on your pet.
Flea Medications for Cats vs. Dogs
Think flea collars are designed for both cats and dogs? Think again. Many flea collar brands, like Seresto and PetArmor Plus, carry separate products for cats and dogs. You should never put a dog’s flea collar on a cat, and vice versa, because they may react negatively to the species-specific medications. For example, the insecticide permethrin is considered safe for dogs but is toxic for cats.
Testing, Evaluations, and Vet Approval
The best flea collars are vet-approved and tested for safety. Keeping up with the latest evaluations and reviews helps you make informed purchasing decisions that impact your pet’s life.
In 2021, the Environmental Protection Agency (EPA) conducted a review of Seresto pet collars, which included an analysis of the reported pet incidents associated with the brand between 2016 and 2020. The EPA reported that, in several cases, pets showed neurological symptoms that appeared to be connected to wearing the collar. Elanco, the registrant of Seresto collars, agreed to continued evaluations and to improve its products’ safety features.
Seresto flea collars are still on the market today, and tens of thousands of people have written five-star reviews of their products, reporting flea-free pets. The mix of positive and adverse reactions shows that each pet can have a unique response to flea medicine. Pet owners should always carefully monitor their cat or dog undergoing treatment.
If you have questions about the safety of a specific flea collar brand, ask your veterinarian if they would personally recommend that brand.

Image via Wendy Toth
Side Effects
Recognizing the side effects of flea collar medication allows you to act quickly in case of an emergency. According to the EPA, pet parents should be on the lookout for:
- Skin effects, such as irritation and redness
- Stomach problems, including vomiting and diarrhea
- Neurological symptoms, such as trembling, depression, and seizures.
If your cat or dog experiences any of these symptoms, remove the collar immediately and contact your veterinarian.
6 Tips When Purchasing and Using a Flea Collar
1. Choose the right size for your pet.
Flea collar brands often separate their products not only by species but also by size. Make sure to choose wisely and take note of exact measurements.
2. Avoid buying a counterfeit product.
Popular flea collars are sometimes mimicked and sold under false pretenses. Always buy your pet products from a reputable source and, when in doubt, call the manufacturer to verify your product.
3. Fit it snugly but not too tight.
When putting the collar on your pup or cat, make sure it’s secure and fits well—not too loose, not too tight. You should be able to fit two fingers underneath the collar. Check periodically to make sure it continues to fit well.
4. Monitor for any signs of irritation or adverse reactions.
Excessively scratching the collar and other stressful behavior indicate that the flea collar may not work for your dog or cat. Try to figure out what might be the problem—for example, if the collar isn’t fitted properly—and, if needed, contact your veterinarian for a new recommendation.
5. Replace the collar as recommended by the manufacturer.
Flea collars are only effective for as long as it says on the label. Follow the manufacturer’s guidelines and when and how often to replace the collar.
6. Store it safely away from children and other pets.
Exposure to flea collar medication, particularly ones containing insecticide, is especially dangerous for children and pets who don’t have fleas. Ensure the pet wearing the collar is kept separate from others in the house. Also, make sure to always wash your hands after handling the pet and collar.
Alternatives to Flea Collars
For other flea prevention and treatment methods, look into topicals and oral flea medications. As with flea collars, there are plenty of vet-recommended options available.
Topical flea medications are usually applied monthly to the pet’s skin, although some products may advise more or less frequent applications. Some popular topical products include Frontline Plus and Advantage II. While this is a direct way to treat fleas, it may not be the best option for finicky pets. Some pet parents aren’t fans of the messy application process.
Oral flea medications, such as NexGard and Bravecto, usually take the form of chewable tablets and are administered monthly or every few months. While the medication is processed via digestion, many of these products are fast-acting. After the first tablet of any oral flea medication, make sure to observe your pet for any reactions.
Conclusion
A flea collar is a common and often effective way to kill fleas and relieve your cat or dog of irritation. Flea collar medications vary, and pet parents should review a product’s ingredients list and talk to a veterinarian before trying a new collar.
Treating and preventing fleas—whether with a collar, topical drops, or chewable tablets—can significantly improve your pet’s health, happiness, and comfort. Work with your vet to determine the best course of action.
Author Bio:
Hayley Bruning is a writer, editor, content strategist, and pet lover. In addition to Petful, she has worked for Cats.com, Treehugger, Petful, and other brands to produce and optimize content. She lives in New Jersey.