Should I patch-test new skincare, and which ingredients most need it?

Patch-testing a small area first is a sensible habit, especially if your skin reacts easily or you are introducing a more active ingredient. It is commonly suggested before starting vitamin C (which can sting on very reactive skin), niacinamide or bakuchiol if you have reacted to a product before, and higher-strength urea on sensitive or compromised skin. If you have ever reacted to a plant extract such as Centella asiatica, which is classed as a weak sensitizer, patch-testing first is especially wise. Introduce one new active at a time and give skin time to adjust; if irritation persists, consult a doctor. This is cosmetic reference, not medical advice.