“Can I use ○○ with △△?” — the question I get most often, turned into a tool. Tap 2–5 ingredients you use and each pair gets one of three verdicts — Good together / Irritation caution / Don’t overlap — with the reason and an evidence grade. Pick three or more and it builds an AM/PM routine (Mon·Wed·Fri / Tue·Sat nights). “Copy result” saves it as text, and the ingredient codes are added to the URL (e.g. #R,C) so you can share a combination.
Evidence grades and references
References
[1] Seité S et al. Skin Pharmacol Physiol 2005;18(2):81-87 — 0.07% retinol + 3.5% vitamin C for 3 months: thicker epidermis
[2] Martin B et al. Br J Dermatol 1998;139(Suppl 52):8-11 — tretinoin + benzoyl peroxide: over 50% degraded in 2 h under light, 95% in 24 h
[3] Tanno O et al. Br J Dermatol 2000;143(3):524-531 — niacinamide increases ceramide and other barrier-lipid synthesis
[4] Milosheska D, Roškar R. Adv Ther 2022;39(12):5351-5375 — dose-dependent retinoid irritation; start low and pair with moisturizer
[5] Korea MFDS, Regulation on the Review of Functional Cosmetics, Annex 4 — niacinamide (2–5%) and vitamin C derivatives are both listed brightening ingredients
[6] The niacinamide + vitamin C warning comes from 1960s experiments at high heat over long periods (nicotinic acid formation); the current consensus is that it does not apply to room-temperature formulas
※ General principles; results vary with concentration, formula and pH. Vitamin C here means pure L-ascorbic acid (derivatives are gentler). If you are being treated for a skin condition, ask your dermatologist. Updated when new research appears (last updated Sept 10, 2026).
How the verdicts were decided
There are only three reasons two ingredients clash: overlapping irritation (the common one), one breaking the other down (oxidizers like benzoyl peroxide), or a pH mismatch (mostly overstated). Green means none of the three applies, yellow means irritation can stack, red means severe irritation or oxidative breakdown. The evidence grade on each pair is "Study-backed" when there is a human-skin study or a stability experiment using both ingredients, "General principle" when the rule follows from each ingredient's properties, and "Theoretical concern" when it rests on chemical reasoning. The full 66-pair chart and the details are in Skincare Ingredient Compatibility Chart, and choosing ingredients from your skin concern continues in Skin Concern Ingredient Map.
▶ Read next: Skincare Ingredient Compatibility Chart – 12 Ingredients, What to Layer and What to Split
▶ Ingredient encyclopedia ①: Retinol: Start at 0.1% – How to Read the Concentration Numbers
▶ 한국어: 성분 궁합 검색기 (Korean version)
Missing an ingredient or a pair? Leave a comment — the most-requested ones get added first.
MS in Veterinary Medicine, PhD in Life Sciences. I worked as a researcher at the Korea Research Institute of Bioscience and Biotechnology (KRIBB) and now run Beaurea, a cosmetics manufacturing company, where I write the formulas myself. This blog reads the research papers and the regulatory documents and turns them into plain-language criteria for choosing ingredients. I don't recommend specific products here – only criteria that apply to any product.
※ This tool gives general principles; results vary with concentration, formula and pH, and with your skin. It does not recommend or criticize specific products. If you are being treated for a skin condition, ask your dermatologist before use. The author runs a company that manufactures and sells cosmetics.