5 Skin-Care Ingredients You Should Never Mix

Her go-to to use in combination with a retinol-containing product is SkinMedica HA5 Rejuvenating Hydrator ($184), which is a hydrator with a gel-like consistency that contains five different types of hyaluronic acids.

Don’t Mix: Vitamin C and Retinol

Retinols and retinoids are powerful ingredients that address several skin-care concerns, from diminishing fine lines and wrinkles and preventing acne to lightening brown spots and increasing collagen production. This is why you’ll find a retinol-based product in many dermatologists’ skin-care routines.

“The main concern with retinol-based products is skin irritation,” Dr. Imahiyerobo-Ip says. “There are several active ingredients that may increase the chance of irritation with retinoids, one being vitamin C.” Like retinol, vitamin C is effective in combating the signs of aging, including fine lines, firmness, and uneven skin tone. However, when these products are used together, they may cause irritation in those with sensitive skin.

Mix: Vitamins C and E

Since both of these vitamins are antioxidants, they become even more powerful at protecting your skin from free radicals in the environment and reducing signs of aging when combined. So, basically, when you use them both, along with a broad-spectrum sunscreen, you’re giving your skin the most optimal shield against the sun and, inevitably, the wrinkles that come along with UV exposure.

If you’re looking for a treatment that combines vitamin C and E, try the Derma E Vitamin C Concentrated Serum ($24).

Don’t Mix: Alpha Hydroxy Acids and Retinol

Glycolic acid and lactic acid are both chemical exfoliants that help improve the skin’s texture, minimize pore size, and also help with brown spots, whereas salicylic acid is a beta hydroxy acid that controls oil production to help prevent pimples. Glycolic acid, lactic acids, and salicylic acids are often found in face washes in low concentrations.

“While these products can be used with retinol-based products, you do not want to use these products one right after the other, particularly if you have sensitive skin,” Dr. Imahiyerobo-Ip says. If you must use acid washes, she suggested using them in the morning and using your retinol-based products at night followed by a gentle moisturizer.

Word to the wise: never start pairing immediately, no matter which product combinations you’re using. “Always allow your skin to adjust to one product for at least a week before you add on a second product to prevent any confusion about potential allergic reaction should you experience one,” Rachel Nazarian, MD, dermatologist at Schweiger Dermatology Group in New York City, says. “It’s slightly more relevant for those with sensitive skin who are more prone to develop allergies to products, but it can happen to anyone, so expose your skin to one product at a time.”

If your skin starts to react badly, you’ll know which product was the most likely culprit. She suggested that those with dry skin avoid exfoliative and acidic pairings, which can cause worsening of dryness, and focus more on gentle antioxidant regimens and moisturizers or oil-based products. People with more oily or acne-prone skin can handle most regimens, but are better suited toward products containing salicylic acid and vitamin A, which help decrease oil-gland production.