It's good to know you keep your pet up with Frontline. Hopefully it's from your veterinarian, as I've had clients bring in Frontline that was not really Frontline. There are lots of other causes of itching. As far as bare spots go, it sounds as if you should see your veterinarian. He/she should be able to help resolve those as shampoos and other over the counter meds rarely fix the causes of hair loss. It is likely he'll need antibtioics. As for Benedryl, it is only helpful in 30% of the cases and the hair loss cause can get worse in the meantime, so please see your veterinarian. Typically if a client wants to use Benedryl, they can use 1mg/lb twice a day on their dog - ie in this case 75 mg (usually 3 tablets of human adult diphenhydramine) AM and PM. Again, please see your veterinarian first, as Benedryl will not make the hair loss any better and often the underlying cause continues with more hair loss resulting.
Itchy skin indicates a hypersensitivity reaction, and the 3 most common causes are parasites (fleas or microscopic mites such as scabies), food allergy, or pollen/dust allergies. The areas of hairloss could be due to the scratching/chewing, or could indicate a secondary skin infection as Dr. Mathis points out, so a visit to the veterinarian is recommended. Make sure that all your dogs are on a good monthly flea preventative such as Frontline, Advantage, or Revolution. If the itch persists despite parasite control and symptomatic treatment with antihistamines, then the underlying allergic cause needs to be identified and treated, starting with a hypoallergenic diet trial for possible food allergy, and if itch persists despite diet change, referral to a veterinary dermatologist for allergy skin testing and desensitization injections for pollen/dust allergies.
Kimberly Coyner, DVM DACVD