What skincare ingredients should not be mixed?

The most important combinations to avoid are retinol with AHA/BHA acids (increases irritation), vitamin C with AHA/BHA acids (can cause over-exfoliation and irritation), and benzoyl peroxide with retinol (they can deactivate each other). Additionally, two strong exfoliants should not be used simultaneously. When in doubt, use one active per routine and separate conflicting actives to morning versus evening.