Do substances such as medicine applied to the skin for treatment purposes prevent ablution?

If a cream or similar substance applied to a part of the body that must be washed during ablution forms a layer, and washing it does not interfere with the treatment, then that part must be washed. If washing would cause harm, it is sufficient to wipe over it with a wet hand. If wiping is also harmful, then it is omitted. (Ibn Abidin, Radd al-Muhtar, 1/102-281) If these substances do not form a layer that prevents water from reaching the skin, they do not affect the validity of ablution.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu