My 18lb dog is borderline. Which Seresto collar size is best?

I would not recommend it. It would be difficult to ensure that the appropriate amount of medication is coming from the collar so that the dogs are protected. You should buy two collars in the appropriate sizes.

The Seresto collar is available for any dog under 18lbs. You will require the small dog collar. These collars last eight months and kill fleas and ticks. In my opinion, they are the best thing around for prevention. This is particularly important for your, as you are living in Reading, which has an extremely high population of lyme-infected deer ticks. If you plan on allowing your dog to be submerged in water for any length of time, you should remove the collar. If you do not, the collar is still good but will only last for five months.

It would not be recommended to do this, as a Seresto collar and Advantage II have the same active ingredients - so you would essentially be giving your dog a double dose of medication, which may not be safe. It shouldn't be necessary to use two different flea control medications as long as you are using an appropriate size/dose for your dog's weight. If you are already using one of these products and are finding that it is not effective, I would recommend switching to a different flea medication entirely, such as Nexgard or Bravecto - these are both much more effective in my experience.

Seresto will be most cost-effective. It is a good quality product and lasts for 8 months. It would be a good choice for a working dog. Nexgard is as good (safe and effective) as Seresto but it will cost you more per year compared with Seresto.

Seresto collars are great for fleas and ticks for up to 8 months which is great! I would make sure she is also on heartworm prevention as this is spread by mosquitoes. I have attached some information about seresto collars https://www.petbasics.com/dog-products/seresto-for-dogs/product-detail/

So Flumethrin is used to kill ticks, and Imidacloprid is used to kill sucking insects such as fleas. This collar is safe to use on Oscar, but like any treatment, there are potential side effects. These include slight hair loss and irritation at the site of the collar. Seresto have been vigorously tested for safety, so you can use it on him without worry. I hope this helps!

The only decent flea collar is seresto. It is labeled for 8 months but the more you bathe them the shorter that period seems to be. So realistically you can get 5-6 months out of it. Bravecto tablets work well as it can last 3 months