Second to picking formulas for your skin type and concerns, it's important that all of the products in your routine complement one another so you can actually see results. 

"Mixing ingredients without proper knowledge of how these ingredients work and what other ingredients they may interact with will be not only a waste of money but also time. It can also lead to frustration if less than expected results are seen (or if the skin becomes irritated)," Dr. Shari Marchbein told us.

Skin irritation is another big factor to consider when curating the product lineup in your regimen. 

"Your skin-care routine should include products that complement each other in order to avoid over-drying, over-exfoliating, or irritating the skin," added Dr. David Lortscher. "More is not always better."

Meet the expert

• Dr. Shari Marchbein is a board-certified dermatologist in New York City.
• Dr. David Lortscher is a board-certified dermatologist and the CEO of Curology.

Keep reading for our expert-approved guide to the dos and don'ts of mixing skin care ingredients.

Retinol

• What It Is: Vitamin A derivative
• Benefits: Promotes skin cell turnover, which can help improve the appearance of fine lines, wrinkles, uneven skin texture, dark spots, and acne
• Side Effects: Irritation
• Works Well With: Moisturizing ingredients like hyaluronic acid and ceramides, as well as SPF
• Don't Mix With: Vitamin C, benzoyl peroxide, and AHA/BHA acids

How to Use

When incorporating retinol into your skin-care routine, make sure to moisturize. 

"Humectant ingredients like hyaluronic acid can draw and hold water molecules to the surface layers of your skin, while oil-based emollient ingredients help seal in moisture," explained Dr. Marchbein.

It's also important to wear sunscreen, in general, but especially if you're using retinol. 

"Many other ingredients we apply to our skin, including retinol and retinoids, can make the skin more sensitive to the sun," added Dr. Marchbein.

What to Avoid

"Retinol is an effective anti-aging ingredient but can exacerbate skin dryness," explained Dr. Lortscher. 

AHAs and BHAs are exfoliating, which can dry the skin and cause further irritation if your skin-care routine already includes retinol.

As for benzoyl peroxide and retinol, they're not recommended to use together. 

"They can literally cancel each other out, rendering them less effective," said Dr. Marchbein. 

Finally, because vitamin C protects skin from environmental aggressors and retinol repairs and rebuilds skin, they're best used at opposite times of the day.

Vitamin C

• What It Is: Antioxidant
• Benefits: Brightens the skin and can even lighten dark spots
• Side Effects: Can irritate sensitive skin or if used in high concentrations
• Works Well With: Antioxidants and SPF
• Don't Mix With: Retinol

How to Use

"Vitamin C protects the skin from oxidative free radical damage and works best in the morning," said Dr. Marchbein. 

When vitamin C is used with other antioxidants like vitamin E, it can boost results and efficiency. The same goes for wearing vitamin C under sunscreen. 

"Vitamin C serums should always be layered under sunscreen because they complement one another and will protect skin against UV damage," explained Dr. Marchbein.

What to Avoid

In contrast to vitamin C, retinol and retinoids build collagen and help repair the skin, so they're best used overnight. Since vitamin C thrives in the daytime, it's best to keep these ingredients separate from each other because they have such different functions.

AHAs/BHAs

• What It Is: Exfoliant (e.g. salicylic, glycolic, and lactic acids)
• Benefits: Improves skin texture, tone, and, in the case of salicylic acid, treats acne
• Side Effects: Can dehydrate and irritate the skin
• Works Well With: Moisturizing ingredients and SPF
• Don't Mix With: Vitamin C and retinol

How to Use

"Moisturizing after applying AHA and BHA is extremely important so as to limit irritation. Look for ceramides, petrolatum, hyaluronic acid, and glycerin to hydrate and soothe skin," advised Dr. Marchbein. 

Using a product that combines multiple low-level AHA and BHA acids can be an extremely effective way to exfoliate and unclog pores.

Like retinol, AHA/BHA acids can cause sun sensitivity. While you should be wearing sunscreen every day regardless of what products are in your skin-care routine, it's extra important to not skip this step when you're using these ingredients.

What to Avoid

"I strongly caution those also using retinoids for acne or anti-aging as the combination with various acids may cause excessive skin sensitivity, irritation, and redness. In fact, AHA and BHA should not typically be used together with retinoids on the same day," explained Dr. Marchbein. "Also, be careful combining various acids or even physical and chemical exfoliants, as this can lead to irritation and even eczema."

Benzoyl Peroxide

• What It Is: Anti-bacterial
• Benefits: Helps to fight acne
• Side Effects: Dryness and irritation
• Works Well With: Gentle hydrating ingredients, SPF, and topical antibiotics
• Don't Mix With: Retinol; if combining with acne prescription tretinoin, use with caution.

How to Use

Benzoyl peroxide can be a game-changing addition to your skin-care regimen if you have acne-prone skin. The caveat? It's another drying ingredient. 

"Because acne treatments, in general, can cause dryness and irritation of the skin, combining them together needs to be done with caution, and every other part of the skin-care routine (i.e. cleanser and moisturizers) need to be extremely gentle and ultra-hydrating, respectively," explained Dr. Marchbein.

Along with moisturizing ingredients that can buffer the dehydrating effects of benzoyl peroxide, the acne-fighting component can be used in conjunction with prescription topical treatments like clindamycin. SPF should also be worn every day.

What to Avoid

As previously mentioned, benzoyl peroxide and retinol can deactivate one another when used together. While prescription acne treatments can be used with BP, tretinoin requires extra care.

"Depending upon how the product is formulated, benzoyl peroxide may inactivate tretinoin somewhat if they are mixed together in the same bottle. They do appear to work just fine in our experience when applied to the skin one after the other — and it does not matter in which order, just rub one product in gently and completely before applying the other," Dr. Lortscher explained. 

He continued, "If you want to minimize any chance of interaction if you are using tretinoin, apply the tretinoin-containing formulation in the PM, and use your benzoyl peroxide in the AM, or use a wash-off benzoyl peroxide cleanser rather than layering a leave-on benzoyl peroxide."

Niacinamide (aka vitamin B3)

• What It Is: Antioxidant
• Benefits: Brightens skin, evens out discoloration, and reduces inflammation
• Side Effects: N/A
• Works Well With: (Almost) every ingredient in your skin-care routine
• Don't Mix With: Vitamin C

How to Use

"Because niacinamide is anti-inflammatory, the skin reacts very minimally to it, and side effects such as irritation are unusual," Dr. Lortscher noted. "It should be compatible with most other skin care products, and for best results, use a leave-on product such as a moisturizer."

What to Avoid

Although they're both antioxidants, vitamin C is one ingredient that's not compatible with niacinamide. 

"Both are very common antioxidants used in a variety of skin care products, but they should not be used one right after the other," said Dr. Marchbein. "Their potency is significantly diminished when used together unless the application is spaced by at least 10 minutes between each serum."

SPF

• What It Is: Topical sun protectant
• Benefits: Helps to prevent skin cancers, wrinkles, and sunspots
• Side Effects: N/A
• Works Well With: Every ingredient in your skin-care regimen
• Don't Mix With: Makeup or moisturizer

How to Use

If you're going to use one skin care product, make it SPF. It's the only way to effectively protect skin from cancer and environmental aggressors, which can lead to premature signs of aging. Given its importance, SPF can be layered over any skin care ingredient.

What to Avoid

SPF can feel like an extra step in an already-extensive skin-care routine, but don't try to take shortcuts. 

"Don't mix your sunscreen with your makeup or moisturizer. Sunscreen should be applied as a single layer to preserve the protective factors," said Dr. Lortscher.