Seborrheic dermatitis (SD) is often seen as a condition that should be treated locally, directly at the surface. On this blog we have discussed a number of such treatments: sea salt, selenium sulfide (in Selsun), zinc oxide and zinc sulfate (in Avene Cicalfate), zinc pyrithione (in DHS Zinc Shampoo), and piroctone olamine (in Sebclair/PromiSeb). These products can definitely be very efficient and many people have reported great results. However, products applied topically (on the surface of the skin) only affect the top layers of the skin. And, as we know, those layers are shedding away constantly. To achieve positive and permanent improvements, the skin needs to be affected from the…