If I want to simplify my beauty routine, or for other reasons, I sometimes mix my products together. For example, I will mix 10% benzoyl peroxide gel with my normal moisturizer for a 2.5% anti-acne medication. Or, I add a couple drops of pure salicylic acid to a benzoyl peroxide product. Occasionally, I add some milk of magnesia 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. But I'm wondering: 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.
Lizzie, via email
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 canóbut it doesn't mean you should, or at least no more than if you were to add some lavender oil to your hair dye to reduce the ammonia-like odor. 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, 2005). Furthermore, how do you figure that adding a 10% benzoyl peroxide product to a moisturizer will result in a 2.5% concentration? It might, but unless you break down the formula in a lab, the final percentage is a guess, even if any benzoyl peroxide survives.
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. Mixing an oil-absorbing agent with a moisturizer is sort of like thinking you're eating healthy if you sprinkle some flax seeds on your doughnut.
As for adding ingredients to sunscreens, yes, doing so will disrupt the level of protection it was designed to supply. You can "counteract" this by mixing in a sunscreen with a higher SPF number, but the bottom line is that if you do so routinely, it will keep you in the dark about just how much sun protection you're getting. Adding SPF 50 to an SPF 8 product does not net SPF 58, though the protection provided is certainly better than applying an SPF 8 alone. Adding salicylic acid to a sunscreen is a problem, again because of the pH factor. Sunscreens need a pH range of 5 to 8 to be most effective. Given that salicylic acid requires a much lower pH range to be effective, adding it to a sunscreen does nothing but lower the level of protection the sunscreen should provide, and that's not good news for your skin. The only type of mixing I generally encourage is when you have a need for a richer moisturizer, perhaps due to seasonal dryness. In that case, it is fine to add a few drops of glycerin and, say, 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.
Source: Paula Begoun