Fleas are easy to miss. A dog scratches a little more than usual, chews at the base of the tail, or can’t settle at night, and most owners think of dry skin or allergies first. One owner put it this way in an Amazon review: “He was itching so frantically and I just thought it was allergies. Never occurred to me it was fleas.” Another wrote: “There’s been days I would find 6 in the house.”

This guide is for dogs only. It lines up five popular over-the-counter flea and tick products by dog weight, explains the difference between a collar and a monthly topical, and covers the label rules that matter most in a home with cats or puppies.

Short version: Weigh your dog, buy the product and size made for that weight, read the whole label before you open the package, and ask your vet which option fits your dog, especially for puppies, seniors, pregnant or nursing dogs, or a dog on other medication.

Collar or Monthly Topical?

• Collar. Seresto collars are worn continuously, and the maker says one collar lasts up to 8 months. Good for owners who forget monthly doses. You need a dog who will wear a collar all the time and a fit you check regularly, especially on a growing puppy.
• Monthly spot-on (topical). Frontline Plus, K9 Advantix II and the Amazon Basics treatment are liquids you apply to the skin between the shoulder blades once a month. Each pack is sized by weight band, so the size you buy has to match your dog. The makers list them as waterproof 24 hours after application.
• Prescription options. Your vet may also suggest a prescription product. That is a conversation worth having before you buy anything, and it is the main reason this guide keeps saying “ask your vet.”

Which Product Fits Which Dog

Frontline Plus, K9 Advantix II and the Amazon Basics treatment are also sold in other weight bands. If your dog is outside the range above, buy the version labeled for your dog’s weight, never a bigger size split between dogs.

The Picks, Up Close (Smallest Dogs to Largest)

Seresto Small Dog Flea & Tick Collar (dogs under 18 lbs)

Seresto Small Dog Collar — Check price on Amazon

For small dogs, the Seresto small collar is the set-and-forget option. Elanco’s listing says it kills and repels fleas for 8 continuous months, starts working within 24 hours, and kills and repels by contact, so fleas do not have to bite your dog to die. It is labeled for dogs at least 7 weeks old and up to 18 pounds, and no prescription is needed. Fit it snugly enough that it can’t slip over the head, and check the fit regularly on a puppy who is still growing.

Best for: Small dogs and puppies (7 weeks and up) whose owners don’t want to track monthly doses.

Seresto 8-Month Flea & Tick Collar for Large Dogs (over 18 lbs)

Seresto Large Dog Collar — Check price on Amazon

The same collar sized for bigger dogs. The listing says it is for dogs at least 7 weeks old and over 18 pounds, with up to 8 months of flea and tick protection from one collar, and Elanco calls it the number one veterinarian-recommended flea and tick collar, based on its own data. It is non-greasy, which suits households with kids who hug the dog. Trim any extra length after fitting, and take it off if your dog’s skin under the collar looks irritated, then call your vet.

Best for: Medium and large dogs who wear a collar all day.

Frontline Plus for Dogs, Large Dog (45 to 88 pounds), 6 Doses

Frontline Plus Large Dog — Check price on Amazon

Frontline Plus is the long-running monthly spot-on. The maker says it protects against fleas and ticks for a full month, kills adult fleas, flea eggs, flea larvae, ticks and chewing lice, and is waterproof 24 hours after application. This pack is for dogs 45 to 88 pounds and holds six monthly doses. Frontline lists it for puppies 8 weeks and older weighing at least 5 pounds, and the dog version says do not use on cats; Frontline sells a separate cat formula.

Best for: Dogs 45 to 88 lb, and owners who prefer a monthly topical to a collar.

Amazon Basics Flea and Tick Topical Treatment for Large Dogs (45 to 88 lbs), 3 Doses

Amazon Basics Large Dog Topical — Check price on Amazon

The budget monthly topical. The listing names the active ingredients as fipronil and (s)-methoprene, says it starts working within 24 hours, and lists up to 30 days of protection per dose, including against deer ticks. It is waterproof 24 hours after application. The label says to use it only on dogs and puppies over 8 weeks, never on cats, and to separate the treated dog from other dogs and cats for 24 hours. Owner reviews are more mixed than for the other picks, so it is worth asking your vet whether it suits your dog.

Best for: Dogs 45 to 88 lb on a tighter budget, in homes where you can keep the dog apart from cats after a dose.

K9 Advantix II Extra Large Dog Flea, Tick & Mosquito Treatment (dogs over 55 lbs), 2-Month Supply

K9 Advantix II XL — Check price on Amazon

For the biggest dogs, K9 Advantix II adds mosquitoes and biting flies to the list. Elanco’s listing says each 30-day dose protects dogs over 55 pounds from fleas, ticks, mosquitoes, biting flies and lice, kills fleas within 12 hours and is waterproof after 24 hours, with no prescription needed. Your dog should be at least 7 weeks old. The label is clear: do not use on cats or other animals. If you have a cat, ask your vet about this one before buying.

Best for: Large dogs over 55 lb who spend time outdoors, in homes without cats or where your vet has OK’d it.

How to Use a Flea Product Safely

1. Weigh your dog first. Use a recent vet weight or a bathroom scale (weigh yourself holding the dog, then subtract your own weight).
2. Buy one product for that weight band. Don’t combine two flea products or split a large dose between small dogs.
3. Read the whole label before opening it. Check the minimum age, weight range and cat warnings, and follow the application steps exactly.
4. Keep cats and kids away while it dries. Follow the label’s waiting time for topicals.
5. Write the date down. A phone reminder for the next dose or collar change keeps protection from lapsing.

Treat the House, Not Just the Dog

• Wash dog bedding, blankets and throw covers in hot water.
• Vacuum floors, rugs and furniture where your dog rests, and empty the vacuum outside.
• Check other pets in the home. Cats need a cat-labeled product; ask your vet. If your cat is overgrooming, see why cats overgroom.
• Use a flea comb during regular brushing so you notice fleas early. Our at-home dog grooming tools guide covers the brushing routine.

FAQ

How do I know if my dog has fleas?

Part the fur near the base of the tail and on the belly and look for small dark specks or moving fleas. A flea comb helps. Specks that turn reddish on a damp white paper towel are often flea dirt. If you are not sure, your vet can check.

Can I use a dog flea product on my cat?

No. Several products on this page say on the label not to use them on cats. Use only a product labeled for cats, and ask your vet which one.

Collar or topical: which is better?

Neither is right for every dog. A collar suits owners who want one product for months; a monthly topical suits dogs who won’t wear a collar all the time. Your vet can help you choose, and may suggest a prescription option instead.

Do dogs need flea prevention in winter?

Fleas can survive indoors in heated homes, and ticks can be active on mild days. Ask your vet whether year-round prevention makes sense where you live.

Can I bathe my dog after applying a topical?

The topicals on this page are listed as waterproof 24 hours after application. Check your product’s label for bathing and swimming instructions.

Final Take

Start with your dog’s weight and your vet’s advice, then pick one product labeled for that weight. The Seresto collars are the long-lasting option for small and large dogs, Frontline Plus is the familiar monthly topical for 45 to 88 pounds, the Amazon Basics treatment is the budget topical for the same range, and K9 Advantix II covers dogs over 55 pounds who also face mosquitoes and biting flies. Keep every one of them away from cats, follow the label to the letter, and treat the bedding and floors along with the dog.