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Can you tell me what breed of dog you have?
How long has it been going on?
What type of shampoo did you use?
Has a skin scraping been done on the dog?
Thank you for that information. Dogs can have airborne allergies that can cause them to scratch , fleas are not the only reason a dog scratches. Generally you may see more scratching , maybe some foot licking, some rubbing of the face along the floor or furniture, but just the scratching with no visible sign of fleas would lead to suspect allergies either airborne or a flea allergy to dry skin. If there is a flea allergy you would most likely see the dog attacking the backside and loosing hair, with dry skin the dog may scratch and in which case you would want to add some omega fatty acids such as Derm caps or fish oil gel tabs to the daily dinner to strengthen the skin and coat. For allergies you would talk to your vet about the use of benadryl. Allergy tests can be done on a dog, but it is expensive and there is no cure, just management
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Here is more on allergies, not all dogs loose fur unless they have been scratching for a while.
Omega fatty acids