If you're anything like me, you pile at least three skin-care products on your face each morning. Right now, my routine includes a moisturizing essence (which feels like water on your skin), an antioxidant vitamin C serum (my current fave is SkinCeuticals C E Ferulic), and a sunscreen with SPF 30 or higher (like Obagi Professional-C Suncare SPF 30). If you don't already layer your skin care, it's a good idea to start: Dermatologists say it traps more active ingredients against your skin, so every product you use works better. But before you get all mixologist on your face, you should know that some skin-care ingredients do not play nice together. Here's your do-not-mix list:
Retinol and alpha hydroxy acids, like glycol and lactic acids. "This duo is a recipe for redness and irritation," says Ranella Hirsch, a dermatologist in Boston. If you do mix them, the fastest way to calm skin is by popping an anti-inflammatory, like aspirin, and smoothing on hydrocortisone cream.

Citric acid with anything. We know what you're thinking: What's citric acid? "It's an ingredient in lots of moisturizers, and when you layer it with other products, it causes irritation," says Hirsch. (It's listed as citric acid on the ingredient list, so it's easy to spot.)

Retinol and acne treatments. Using salicylic acid and retinol at the same time causes dryness and redness. Mixing benzoyl peroxide with retinol has the opposite effect: "The two ingredients have been shown to deactivate each other," says Fredric Brandt, a dermatologist in New York City and Miami.

Vitamin C and alpha hydroxy acids. "They change the pH of vitamin C enough to destabilize it, making the antioxidant ineffective," says David Bank, a dermatologist in Mount Kisco, New York. If you're wearing a serum with glycolic acid, pick serums with heartier antioxidants, like green tea and resveratrol.

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