How long does it take for those flea ampoules to start being effective on dogs?

Flea bites are extremely itchy so that may be why the dog is still scratching. The flea stuff starts working within minutes.

Fleas need to be dealt with immediately and aggressively. If you Google the life cycle of a flea you will learn how they multiply and survive.

Googling fleas is what brought me to fluther over 7 years ago! I had fleas twice – once in house when my daughter was only months old and about 3 years later in the yard and on the deck. Both times I sought the help of professionals.

Most topical applications kill the flea when it bites, so it’s not going to stop his itching until the flea population is gone. (Please, please, never use over the counter flea treatments, especially Hartz. We see so many toxicity cases.)

As far as I know, once your dog has fleas, you need to deal with his bedding, carpets, etc. to effectively kill off the population. Ask your vet, maybe?