The difference between the two kinds of frontline is the active ingredients and consequently, the effect seen on the flea population.
Frontline Top Spot has as an active ingredient only fipronil which is an insecticide. So this product is effective only against adult fleas and does not break their life cycle and so new generations continue to hatch.
Frontline Plus has as active ingredients fipronil (same action as above) and S-methoprene. The second substance is an insect growth regulator and is effective against immature stages of fleas. It stops them from hatching so no new generation of fleas appear as long as your dog is protected with this product.
Please use the correct product depending on your pet’s body weight. Always read the information enclosed with the product and follow the manufacturers’ advice.