If I want to simplify my beauty routine, or for other reasons, I sometimes mix my products together. For example, I will mix 10 percent benzoyl peroxide gel with my moisturizer for a 2.5 percent anti-acne medication. Or I add a couple drops of pure salicylic acid to a benzoyl peroxide product. Occasionally, I add an oil-absorbing mask to my sunscreen or to my nighttime moisturizer if I want to control oil, or I add a couple drops of glycerin if I find a product is too drying.
Can I do that? I mean, can salicylic acid and benzoyl peroxide coexist in one product? Does adding something to sunscreen mess up its protection? Can that be counteracted by adding a squirt of, say, a sunscreen with SPF 50? If a toner had a good pH, and I added a couple drops of salicylic acid, would it still be stable? I haven’t tried all of these yet, but am quite enjoying being a cosmetic chemist in my own bathroom.
A: Although I admire your ingenuity in thinking about cosmetics, I must advise that what you’re doing is indeed affecting the stability, performance and safety of the products you’re experimenting with. The answer to the question of “Can I do that?” is yes, of course you can - but it doesn’t mean you should. Cosmetics chemistry is as much art as it is science. Adding ingredients to finished products in an effort to create an enhanced version or to alter the original function won’t necessarily blow up in your face, but you are negating the effectiveness of the very ingredients that could be helping your skin.
For example, salicylic acid (BHA) must be in a base that has a certain pH range if it is going to exfoliate skin. Mixing it into a product with a pH above that range will not produce the results you’re looking for, and may cause undue irritation. According to the chemists I spoke to, salicylic acid and benzoyl peroxide don’t coexist well in the same product, because they need different pH ranges for optimal effectiveness. In addition, the FDA does not permit both active ingredients in one product, which explains why no company is making a BHA/benzoyl peroxide solution (Source: Code of Federal Regulations, Title 21, Volume 5, April 1).
Milk of magnesia is an effective absorbent for excess oil, but adding it to a moisturizer defeats the purpose because it also “absorbs” the oil-soluble ingredients in the moisturizer, which in turn reduces its oil-absorbing ability on your skin.
As for adding ingredients to sunscreens, yes, doing so will disrupt the level of protection it was designed to supply (in fact, when a manufacturer changes even one ingredient in a sunscreen formula the FDA mandates retesting it from scratch).
The only type of mixing that can work safely and be of benefit is when you have a need for a more emollient moisturizer, especially during winter or in arid environments. In that case, it is fine to add a few drops of glycerin, olive or jojoba oil, to your regular moisturizer. However, to avoid affecting the stability of the ingredients in the entire container of product, be sure to do the mixing in your hand, or apply the oil over the moisturizer directly onto your skin.
