The short answer is days to weeks. But there is a lot to learn about different products on the market and about the active and inactive ingredients they contain. Let’s start by defining what makes a “medicated shampoo.” In the 1980s the FDA wanted to weed out the snake oils of the shampoo world and so all products that were purported to help dandruff, seborrheic dermatitis and psoriasis were asked to submit data from clinical trials supporting the claims they were making. When it came to treating seborrheic dermatitis, the following ingredients at certain percentages made the cut: pyrithione zinc, selenium sulfide, coal tar, and salicylic acid . Later the active ingredient ketoconazole was added to the list. When I say, “made the cut,” it means these ingredients (formally called “Active Ingredients”) had to prove they were both safe and effective. These 5 ingredients when made at the required percentage, with the appropriate labeling, and using factories that follow stringent manufacturing processes are called OTC (Over The Counter) medicated shampoos. Medicated Shampoo Labels are important When purchasing an OTC medicated shampoo for seborrheic dermatitis, you will notice a familiar appearing label on the back of the bottle or packaging called a “Drug Facts” label. This is also where you can easily identify which Active Ingredient is featured in each medicated shampoo. Some medicated shampoo brands make different product lines with different active ingredients. Read that label! A few lines down from the top of that label you will find the subheading “Directions” and then the following lines that say something along the lines of, “For best results use at least twice a week or as directed by a doctor.” You may wonder why there are even different products on the market if the they all say the same thing? When claims for these active ingredients were evaluated in the 1980s, the experts weighed in and decided that if used with that frequency (two times per week) after a couple of weeks people would indeed see less flaking, scaling and irritation associated with seborrheic dermatitis. So yes, all OTC medicated shampoos do work to fight flakes. So what distinguishes one medicated shampoo from another? People react differently to different active ingredients. For example, one person may not like what their skin feels like with salicylic acid, while another person finds the smell of coal tar or selenium sulfide offensive. Some may find pyrithione zinc to be too gentle or ketoconazole to be too drying. Each brand of shampoo uses different active ingredients, and can do so at different percentages of active ingredient to deliver the active ingredient to your scalp and skin and accomplish the task at hand to improve the symptoms of seborrheic dermatitis and dandruff. It is similar to Tylenol and Ibuprofen, they both reduce fever and pain, but people have preference on which one works best for them. Inactive Ingredients are important A big distinguishing factor that we have yet to discuss is the inactive ingredient and the importance of what a medicated shampoo does not contain. Inactive ingredients are the crucial vehicles that make the product foam and help to wash away accumulated oils on the scalp and body. Many companies have added “natural” inactive ingredients that help to soothe or soften the skin or to moisturize the hair. And most products have scent or color, as many people find odors and colors a pleasing experience. However, many adults with seborrheic dermatitis will describe themselves as having sensitive skin. Children have sensitive skin. These groups of people will benefit from avoiding products that contain common skin irritants like scent and color. These folks should look for a medicated shampoo that is fragrance free, dye free, paraben free, and sulfate free. If you are an adult reading this article, you may have been surprised to see children mentioned in the paragraph above. Infants get “cradle cap.” Cradle cap is the scaling and flaking that occurs on the scalp and eyebrows. We see it in kids as young as 3-4 weeks old. Cradle cap is an infant version of seborrheic dermatitis. About 33% of kids persist with cradle cap through the age of 5 years old. 70 percent of pediatricians suggest using an OTC Medicated Shampoo to get rid of cradle cap. Until a couple years ago, there was no product created specifically with children in mind. How long does it take to get rid of seborrheic dermatitis? Some people may see results in as few as one or two washes. However, most will see significant improvement after 1 to 2 weeks of use. If there is much scale build up as you find in cradle cap you may need to use a medicated shampoo for seborrheic dermatitis daily for 3 to 7 days (Many OTC shampoos can be used daily as tolerated). People with thick scale build up, should consider leaving the shampoo on for 1-2 minutes before rinsing. People in this latter category should start to see results in 2-3 weeks. Once the scaling and flaking is under control, we suggest tapering use to 2-3 times per week. References: \t358.710 Active ingredients for the control of dandruff, seborrheic dermatitis, or psoriasis. PART 358 -- MISCELLANEOUS EXTERNAL DRUG PRODUCTS FOR OVER-THE-COUNTER HUMAN USE. Code of Federal Regulations. Title 21, Volume 5. Revised as of April 1, 2019. CITE: 21CFR358.710 https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr358.710 . Accessed 9 April 2020.