No, you cannot split a large dog flea treatment on a small dog. That is because small dog flea treatment has a different chemical concentration compared to flea medication for a large dog. These concentrations are not balanced out or divided as you think they are when you split the medication. As a result, you might think you are giving your small dog a small dosage when using a split-up large dog flea medicine, but in reality, you are giving them the same concentration only at a small amount. It will not work and might even make your dog sick.