7 Skin Care Ingredients You Should Never Mix, According to Derms

Although retinol and vitamin C are commonly used ingredients to help improve fine lines, texture, and brown spots, using them at the same time can reduce their efficacy, according to Dr. Pierre. "Retinol and vitamin C are designed to work at different pH levels," he says. "To get the most out of each product, try to use vitamin C in the morning and your retinol in the evening."

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