Excerpt from my suppliers site:
•Frontline Spot On◦This contains one ingredient.
◦It coats the skin and hair after being applied and kills off fleas.
◦It persists on the coat even if the animal swims or is shampooed, as long as there is at least a 24 hour gap before and after applying the Frontline.
•Frontline Combo (Prescription required)◦This contains 2 ingredients.
◦The first one is exactly the same as in normal Spot-on.
◦The second one also coats the skin and coat, but does not kill fleas at all. Instead it sterilises fleas, so that they cannot breed.
◦The idea of it is that it persists longer on the coat than the killing ingredient.
◦Hence if people do not re-apply the frontline as often as they should, then the killing ingredient will no longer be effective (especially on animals at the top of the weight range of the product used). The other ingredient works for about twice as long as the main ingredient. The idea behind this is that if fleas cannot breed, you only get very small numbers of them and they are unlikely to be noticed or do any harm.
Personally I find the ordinary frontline is very effective, and we haven't had an outbreak since I started using it around 7 years ago.
