Don't Mix: Vitamin C and Alpha Hydroxy Acids
Both of these ingredients offer antioxidant benefits, but when combined, they can throw off the pH balance of your skin due to too high levels of acidity. Products containing vitamin C (L-ascorbic acid) as an ingredient contain low levels of acidity (typically a three on a scale of 14). When you use this kind of product along with an alpha hydroxy acid, like glycolic or lactic acid, you're essentially reducing the effectiveness of the vitamin C product. Is it dangerous or irritating to combine them? Probably not. But you'll be wasting a whole lot of what might be an expensive product if you do.
Mix: Vitamin C and Ferulic Acid
Vitamin C is a potent antioxidant that improves fine lines and wrinkles and help to eliminate unwanted hyperpigmentation. This antioxidant becomes even more powerful when it is combined with glutathione and ferulic acid, said Dr. Imahiyerobo-Ip. One of the top serums that combines these two ingredients is SkinCeuticals C E Ferulic Combination Antioxidant Treatment ($165). It synergistically mixes 15 percent pure vitamin C (L-ascorbic acid), 0.5 percent ferulic acid, and one percent vitamin E, which serve together as an environmental shield and antiaging armor for your skin.
Don't Mix: Vitamin B3 and Alpha Hydroxy Acids
Also known as niacinamide, vitamin B3 is a skin conditioning agent that helps repair and refine the skin's structure. It tends to work best in an environment with a neutral pH, which we just explained is certainly not alpha hydroxy acids, as they have an extremely low pH level.
"When used in a product containing high levels of alpha hydroxy acids (i.e. glycolic, lactic,) nicotinic acid is produced, which may result in skin flushing and potential irritation," explained Ramya Viswanathan, product development manager at Biossance. While it won't necessarily harm you to use them both together, they basically cancel each other out.
Mix: Retinol and Hyaluronic Acid
Hyaluronic acid is another powerful humectant that can hold up to 1,000 times its weight in water, meaning it does wonders in hydrating the skin. "Skin loses water and moisture as we age, and especially with the use of drying ingredients such as retinol in other products," explained Dendy Engelman, M.D., dermatologist in New York City. "This ingredient will help store hydration."