There can be many reasons… First of all, the soap used may be too stripping for the skin. If in addition to scratching, the skin is very dry, it is better to opt for a surgras soap which will respect the hydrolipidic film. As for the temperature of the water, it should not be too hot so as not to attack the skin. Once in the shower, you soap without rubbing too much on the skin, whether you use a washcloth or not. Gestures that calm: small gentle circular movements, even if it means ironing several times on the same areas. Then, we rinse well (and everywhere) to make sure that no residue of the cleanser is left on the skin, which can be the cause of the itching. Finally, to dry the body, do not rub the skin vigorously but gently dab it with the towel. Finally, hydrate immediately by applying a hypoallergenic body milk or cream and preferably fragrance-free.
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