Skin-Care Ingredients You Should Never Mix — and Ones You Should

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 (L-ascorbic acid) 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.

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," Dendy Engelman, MD, a dermatologist in New York City, says. "This ingredient will help store hydration."

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.

If you find yourself tempted to combine retinol and salicylic acid, two powerhouses for fighting signs of aging, think again. Combining salicylic acid and tretinoin, for example, is risky. "Retinol is extremely unstable," NYC-based dermatologist, Joshua Zeichner, MD, FAAD, previously told PS. "It doesn't play nicely with a salicylic acid or benzoyl peroxide. Combining two leave-ons, like masks or overnight treatments, can hit the skin too hard if you use them all at one time."

Glycolic acid is a chemical exfoliant in the alpha-hydroxy acids (AHAs) family that helps improve the skin's texture, minimize pore size, and also helps with brown spots. Glycolic acid, as well as 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.

Benzoyl peroxide and salicylic acid are both great ingredients for treating acne. In some cases, using them together in a formulation can make them more effective at banishing breakouts. That said, most experts don't recommend mixing two different products with these ingredients together. This is because in the wrong dosage, the two powerful ingredients can cause skin irritation.

Salicylic acid and vitamin C are two very different ingredients and when used together in the right way, can really help you accomplish the bright, clear skin of your dreams. That said, you shouldn't use them at the same time. It's best to use vitamin C in the morning and salicylic acid at night to avoid irritation.

If you've been dealing with acne for a while, you probably already know that both of these ingredients work to prevent breakouts, and you may find yourself wondering, Can you use benzoyl peroxide with retinol? But there's a good reason benzoyl peroxide and adapalene are not often used in combination in the very same product. "Benzoyl peroxide is a potent acne product that's great for inflammatory acne, but many people are unaware that benzoyl peroxide can inactivate a topical retinol," Dr. Imahiyerobo-Ip says.

This doesn't mean that you cannot use benzoyl peroxide during the same period that you're using retinol; it just means benzoyl peroxide and tretinoin should never be layered on top of each other or you'll be in for a rude (and red) awakening. Dr. Imahiyerobo-Ip recommends using benzoyl peroxide in the morning and a retinoid at night.

Retinol is a miracle skin-care ingredient, but it's also pretty drying. That's why dermatologists and estheticians urge their patients and clients to use it sparingly. It's also recommended to use a bit at a time until skin is accustomed to it and stops peeling from it. One ingredient that can help prevent irritation when using a retinol-containing product is glycerin, a powerful humectant that helps draw water to the skin.

"When used together, they work synergistically to combat the signs of aging while also moisturizing the skin to minimize the irritating effects of retinoids," Dr. Imahiyerobo-Ip says. A great product that combines the amazing benefits of a retinol with the moisturizing benefits of glycerin is Hydropeptide's Nimni Cream ($224).

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 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.

Also known as vitamin B3, niacinamide 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 or lactic), nicotinic acid is produced, which may result in skin flushing and potential irritation," Ramya Viswanathan, product development manager at Biossance, says. While it won't necessarily harm you to use them both together, they basically cancel each other out.

Vitamin C is a potent antioxidant that improves fine lines and wrinkles and helps to eliminate unwanted hyperpigmentation. This antioxidant becomes even more powerful when it is combined with glutathione and ferulic acid, says Dr. Imahiyerobo-Ip. One of the top serums that combines these two ingredients is SkinCeuticals C E Ferulic Combination Antioxidant Treatment ($182). 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 anti-aging armor for your skin.

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 vitamins C and E, try the Derma E Vitamin C Concentrated Serum ($25).

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, a 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.

Jenn Sinrich is a freelance writer, editor, and content strategist in NYC. Her work can be found in PS, Women's Health, SELF, Reader's Digest, Health, PureWow, and more.