Dawn is a strong detergent which would strip the oils in their coats. The medicine is carried through their skin glands. I've used gentle shampoos that are not soap free, but it would be best to use soap free shampoos.
This is from Frontline's website:
Use a gentle, soap-free, pet shampoo. Don't use other insecticides (flea treatment such as flea wash), human shampoo, or anything that strips oils from your pet's coat or skin. By mistake I went to their Australian site, but the information would be the same.
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