Dr. Neal Schultz from DermTV posted this video on Youtube, he states that layering products with the same active ingredient, like two glycolic acid toners doesn’t change the concentration on the skin? This doesn’t make sense to me!
I’m going to assume (and hope!) that people watched Dr. Neal Schultz’s video as he’s a great source of information and gave a great demonstration of adding two products to each other.
He’s correct in that the percentage of active ingredient doesn’t change, but what he doesn’t cover is the effect of layering the product on the skin. The reason why layering two products with the same active ingredient will have a greater effect is because of the total amount of active ingredient applied.
So let’s break it down in to some math…(Sorry!)
It’s Math Time!
In my example, I’ll be using the same two 20% glycolic acid serums.
Let’s say that one application of the serum to the face uses 10 ml of product, since it’s 20% glycolic acid, one application of 10 ml of product will mean you’ve put on 2 ml of glycolic acid (20%). Now if you layer on an additional layer, you now have a total of 20 ml of product on your face, which amounts to 4 ml of glycolic acid. So while the percentage of active ingredient to carrier stays the same (20%), you have double the amount of active ingredient on the skin.
I hope that isn’t confusing, so I’ll use another example. If I have 2 candies, one chocolate and one peanut butter, my chocolate percentage is 50%. If I take an extra 2 candies, another chocolate and another peanut butter, my chocolate percentage is still 50% – but I now have 2 chocolates.
This layering technique is commonly used by dermatologists and cosmetologists to make chemical peels penetrate the skin more and cause a deeper peel.
As a final thought, applying a 20% glycolic acid serum twice may not give you the same results as applying a 40% glycolic acid serum once. The reason is because of the differing pH. A 40% glycolic acid serum will be much more acidic, which will cause it to exfoliate (and irritate) the skin more.