Niacinamide.

What it does

Benefits

• Reduces redness and calms inflammation, which can help rosacea-prone and reactive skin
• Evens skin tone and visibly lightens hyperpigmentation with daily use over about 8 weeks
• Strengthens the skin barrier and improves hydration by boosting ceramide and fatty-acid synthesis
• Helps regulate oil and can reduce the sebum excretion rate, useful for oily and acne-prone skin

Side effects

• Very well tolerated; side effects are minimal at typical cosmetic concentrations (around 2-5%)
• Some people experience mild flushing or tingling, and very high concentrations can be irritating for sensitive skin
• Rare allergic reactions can occur, so patch-test if you have very reactive skin

Who it's for

Almost everyone, but especially people with redness, uneven tone, dark spots, enlarged pores, or oily and acne-prone skin

Who should be careful

No meaningful contraindications at cosmetic concentrations; very sensitive skin should start at lower strengths and patch-test, as high concentrations can occasionally cause flushing or irritation