Ingredient caution: vitamin C/ascorbic derivatives might cause tingling, redness or irritation in sensitive skin; niacinamide is usually gentle but might cause tingling or flushing in very reactive skin; rich oils, butters or occlusive emollients might feel heavy or clog pores in acne-prone/oily skin; preservatives such as phenoxyethanol/benzyl alcohol might sting very sensitive or compromised skin. Do a patch test before applying. First-time users should start 3–4 times a week and avoid layering with strong exfoliating acids if the skin stings. Start slowly if your skin is sensitive and avoid combining too many new actives at once. Use a small amount first and avoid applying on active acne if it feels greasy. Do not apply on broken, freshly waxed or irritated skin. Stop use if irritation persists.