Index / Data guides
The verified answer, in one place
40 guides and growing. Each page answers one high-stakes skincare question directly — concentration, layering, safety — with every claim sourced to the primary literature.
- 17 How to layer skincare actives: what the evidence actually says Two layering decisions are evidence-based: vitamin C belongs in the morning (photostability + photoprotection) and retinol belongs at night (UV degrades it on skin). The rest of the rules circulating online — the pH-waiting rule, the niacinamide-cancels-vitamin-C myth, the 30-minute wait between layers — are largely unsupported convention or outright false. Layering guide · 4 sections · 11 sources 11 sources
- 19 How to treat keratosis pilaris ("strawberry skin") Keratosis pilaris responds to two things together: a gentle exfoliating acid to dissolve the keratin plugs (lactic acid first, salicylic acid to reach the follicle) and ceramides to repair the dry, impaired barrier. It is benign and chronic, so the goal is management, not a permanent cure — and patience plus moisture beats aggressive scrubbing. Data guide · 3 sections · 6 sources 6 sources
- 21 L-ascorbic acid vs vitamin C derivatives: which percentage question are you actually asking? L-ascorbic acid's 8-20% effective range and 20% saturation ceiling — covered in full in our vitamin C percentage guide — only apply to the free-acid form. If your product lists a derivative (sodium/magnesium ascorbyl phosphate, ascorbyl glucoside, tetrahexyldecyl ascorbate), none of that L-AA data transfers; see our vitamin C derivatives percentage guide for those numbers. This guide is the decision layer in between: which form should you actually be comparing percentages within. Concentration guide · 4 sections · 5 sources 5 sources
- 32 Skincare while pregnant: which actives are safe, which to avoid, and what we simply don't know Avoid all retinoids (topical retinol, retinal, retinyl esters, tretinoin). Azelaic acid, niacinamide, hyaluronic acid, and vitamin C are widely considered compatible with pregnancy. Low-percentage salicylic acid spot treatment is generally regarded as low-risk; glycolic/mandelic AHAs in moderation are considered acceptable. Benzoyl peroxide has ACOG support for limited topical use. Several popular actives — bakuchiol, kojic acid, tranexamic acid, alpha-arbutin — simply lack pregnancy safety data; 'no retinoid' does not equal 'proven safe.' This guide is not medical advice: confirm your full routine with your obstetrician or midwife. Pregnancy guide · 5 sections · 10 sources 10 sources
- 07 The best ingredients for acne marks (PIH vs PIE) First identify the mark: flat BROWN marks (PIH) are excess pigment and respond to azelaic acid, niacinamide, vitamin C, and exfoliating acids; flat RED/pink marks (PIE) are dilated vessels and respond better to barrier repair, anti-inflammatory care, and tranexamic acid. Azelaic acid is the most versatile single pick, and SPF is mandatory for both. Data guide · 5 sections · 10 sources 10 sources
- 08 The best ingredients for acne scars and texture First, be honest about the kind of mark. Flat brown or red marks are not scars — they fade with the brighteners in our acne-marks guide. True scars are textural: raised/keloid scars are a medical matter for a dermatologist, and deep atrophic (icepick, boxcar, rolling) scars usually need in-office procedures. Topicals — retinol, vitamin C, centella, and AHAs — genuinely improve general texture and support collagen repair, but they improve the appearance, they do not erase a scar. Data guide · 4 sections · 7 sources 7 sources
- 12 The best ingredients for fine lines and wrinkles Retinoids lead the evidence: retinol is the gold-standard OTC anti-ager and retinaldehyde is the more potent step-up. Peptides and vitamin C are strong supporting actives, niacinamide helps fine lines, and hyaluronic acid only temporarily plumps. The single most effective anti-wrinkle habit is daily SPF, which prevents the damage the others repair. Data guide · 6 sections · 12 sources 12 sources
- 09 The best ingredients for hyperpigmentation and dark spots There is no single best ingredient — match the active to the cause. Vitamin C and niacinamide are the everyday all-rounders, tranexamic acid is the melasma and redness specialist, azelaic acid is best for acne marks (PIH), and alpha-arbutin and kojic acid are direct tyrosinase inhibitors. None of them hold without daily SPF. Data guide · 6 sections · 12 sources 12 sources
- 10 The best ingredients for oily skin and large pores First, the honest part: you cannot physically shrink a pore — pore size is largely genetic. What you can do is reduce how large pores look by clearing the congestion and oil that stretch them. Niacinamide regulates oil and refines appearance, salicylic acid de-congests from inside the pore, and a retinoid refines texture long-term. Over-stripping makes oily skin worse, not better. Data guide · 3 sections · 6 sources 6 sources
- 11 The best ingredients for redness-prone and sensitive skin For redness-prone, reactive skin the goal is to calm inflammation and rebuild the barrier — not to strip or over-treat. Azelaic acid has the strongest evidence for reducing redness, while niacinamide and ceramides repair the compromised barrier that drives sensitivity. Important: persistent facial redness can be rosacea, a medical condition — if it is ongoing, see a dermatologist for a diagnosis and prescription options rather than self-treating. Data guide · 3 sections · 6 sources 6 sources
- 01 What percentage of adapalene actually works? 0.1% is the over-the-counter strength (Differin and its generics) with the deepest clinical trial base — matching tretinoin 0.025% on efficacy while causing far less irritation. 0.3% is reserved for prescription use in moderate-to-severe acne. Concentration guide · 3 sections · 7 sources 7 sources
- 02 What percentage of alpha arbutin actually works? 1–2% is the fully studied and regulatory-endorsed range; the EU SCCS confirmed 2% as both the effective ceiling and the safe-use limit for face products. Concentration guide · 5 sections · 10 sources 10 sources
- 03 What percentage of avobenzone actually works? 3% is the FDA-set legal maximum for avobenzone in US over-the-counter sunscreens, and nearly every disclosed product on the market uses that exact ceiling. Unlike a treatment active, there's no lower 'effective' percentage to chase — the real question is whether the 3% is photostabilized, not how much of it there is. Concentration guide · 3 sections · 8 sources 8 sources
- 04 What percentage of azelaic acid actually works? 15% (prescription, rosacea) and 20% (prescription, acne and melasma) are the concentrations with clinical trial evidence; OTC 10% products are widely used but have no independent RCT data at that exact percentage. Concentration guide · 5 sections · 14 sources 14 sources
- 05 What percentage of bakuchiol actually works? 0.5% is the only concentration tested in a published RCT; it produced meaningful wrinkle and pigmentation improvements over 12 weeks. Products up to 2% are commercially common, but no dose-response data exists to know whether higher concentrations do more. Concentration guide · 6 sections · 5 sources 5 sources
- 06 What percentage of benzoyl peroxide actually works? 2.5% is enough — a 1986 double-blind RCT found it equal to 5% and 10% for reducing inflammatory acne, with significantly less dryness and redness. Higher concentrations add irritation, not effectiveness. Concentration guide · 6 sections · 10 sources 10 sources
- 13 What percentage of copper peptides actually works? There is no human RCT establishing an effective topical percentage for GHK-Cu. The CIR safety review lists typical historical cosmetic use at under 10 parts per million (0.001%), while modern serums market 1% — a thousand-fold higher — and no published study compares the two on human skin. Concentration guide · 3 sections · 3 sources 3 sources
- 14 What percentage of ectoin actually works? 5.5-7% is the concentration behind the controlled human trials showing real barrier and eczema benefit. Most cosmetic ectoin serums sit well below that — only one of three disclosed products in our catalog reaches the clinically studied dose. Concentration guide · 3 sections · 5 sources 5 sources
- 15 What percentage of ferulic acid actually works? 0.5% is the concentration with the clinical evidence behind it — the exact figure in the landmark CEF photoprotection trials and the SkinCeuticals stabilization patent. Every major disclosed product on the market, from the $185 original to budget dupes, uses this same 0.5%. Concentration guide · 4 sections · 8 sources 8 sources
- 16 What percentage of glycolic acid actually works? 5–10% in a leave-on product at pH 3.5–4.5 is the evidence-supported range for home use; 8% is where a 22-week RCT demonstrated significant photodamage improvement. Concentration guide · 6 sections · 11 sources 11 sources
- 18 What percentage of hyaluronic acid actually works? Percentage is the wrong question for hyaluronic acid. Efficacy at 0.1% is clinically validated across all molecular weights — what actually matters is molecular weight and whether you seal it with an occlusive. Concentration guide · 6 sections · 10 sources 10 sources
- 20 What percentage of kojic acid actually works? 1% is the clinically supported floor and the EU regulatory ceiling for leave-on products; the effective range is 1–2%, with no evidence that higher concentrations improve results and clear evidence they increase sensitisation risk. Concentration guide · 6 sections · 13 sources 13 sources
- 22 What percentage of lactic acid actually works? 12% is the concentration with the strongest evidence for dermal-level improvement, but 5% and 8% both have their own controlled trial data. Consumer leave-on products should stay at or below the CIR's 10% maximum, at pH 3.5 or above. Concentration guide · 4 sections · 4 sources 4 sources
- 23 What percentage of madecassoside actually works? The two cleanest human data points are 1% whole Centella asiatica extract (a 280-patient scar RCT) and 0.1% madecassoside co-formulated with 5% vitamin C (a 20-patient photoaging RCT) — real numbers, but thin ones. There is no dose-ranging trial and no validated 'minimum effective %' for isolated madecassoside. Concentration guide · 4 sections · 6 sources 6 sources
- 24 What percentage of mandelic acid actually works? 5–10% works for daily leave-on use; 20–45% is for professional peels only — and mandelic acid's key advantage over glycolic acid is gentler penetration due to its larger molecular size (152 Da vs 76 Da), not a higher top concentration. Concentration guide · 6 sections · 11 sources 11 sources
- 25 What percentage of niacinamide actually works? 2–5% niacinamide covers every validated clinical endpoint; concentrations above 5% add marketing pressure but little peer-reviewed evidence of additional benefit. Concentration guide · 4 sections · 6 sources 6 sources
- 26 What percentage of panthenol actually works? 5% dexpanthenol is the concentration behind the repeated barrier-repair and irritation-protection RCTs, and one dose-ranging study found hydration and barrier benefits kept improving up to roughly 7-11%, with no published data testing meaningfully higher. Concentration guide · 4 sections · 5 sources 5 sources
- 27 What percentage of peptides actually works? There is no single 'peptide percentage' — the studied doses span three orders of magnitude depending on which peptide you mean. Palmitoyl pentapeptide-4 (Matrixyl) has RCT evidence at 3 parts per million; argireline's originating study used 10%. This guide covers signal and neurotransmitter-inhibiting peptides; copper peptides (GHK-Cu) have their own separate concentration question. Concentration guide · 5 sections · 7 sources 7 sources
- 28 What percentage of PHA actually works? Gluconolactone has been directly tested at 10% and 30% as an in-office peel with measurable skin effects at both strengths, and a separate 12-week daily-use regimen delivered anti-aging results comparable to a glycolic-acid (AHA) regimen — without naming a leave-on percentage. Concentration guide · 4 sections · 4 sources 4 sources
- 29 What percentage of retinaldehyde actually works? 0.05% to 0.1% is where every published clinical study sits, including a 2026 head-to-head comparing the two directly. Retinaldehyde is roughly 10 times more potent per unit concentration than retinol, which is why its effective range is so much lower. Concentration guide · 4 sections · 8 sources 8 sources
- 30 What percentage of retinol actually works? 0.3–0.4% retinol has direct RCT evidence for wrinkle and collagen improvement; 0.1% shows measurable photoaging benefit with minimal irritation; going above 1% crosses into diminishing returns and rising irritation. Concentration guide · 6 sections · 13 sources 13 sources
- 31 What percentage of salicylic acid actually works? 0.5–2% is the clinically validated OTC range; 2% leave-on is the benchmark for acne — but none of it works as advertised if the product's pH is above 4. Concentration guide · 6 sections · 11 sources 11 sources
- 33 What percentage of snail mucin actually works? The clinical trial evidence is for SCA (Cryptomphalus aspersa secretion) at 8% and 40% — real RCTs with measured facial outcomes. The '96%' and '92%' on Korean snail-essence labels describe a completely different thing: how much of the formula is filtrate, not a standardized dose of any active. Concentration guide · 3 sections · 4 sources 4 sources
- 34 What percentage of titanium dioxide actually works? 25% is the FDA-permitted maximum for titanium dioxide in US sunscreens, but real products run far lower — 4% to 8% in our catalog — because it's almost always paired with zinc oxide to fill the long-UVA gap titanium dioxide leaves on its own. Concentration guide · 3 sections · 5 sources 5 sources
- 36 What percentage of tranexamic acid actually works? Topical tranexamic acid at 2–5% reduces melasma and PIH; 3% is the most replicated clinical concentration and matched 4% hydroquinone in a split-face RCT — but the strongest overall efficacy evidence for melasma belongs to oral TXA, which is a prescription route with different risks. Concentration guide · 6 sections · 10 sources 10 sources
- 37 What percentage of urea actually works? Urea is unusual: the percentage changes what it does, not just how strongly it does it. Roughly 10% hydrates; 20% measurably outperforms 10% for rough, discolored skin in a head-to-head RCT; and 40% is the clinically validated concentration for calluses, thickened nails, and stubborn xerosis. Concentration guide · 5 sections · 8 sources 8 sources
- 39 What percentage of vitamin C actually works? 8–20% L-ascorbic acid at pH below 3.5 — skin absorption peaks at 20% and plateaus there, so going higher gives you nothing extra except more irritation risk. Concentration guide · 4 sections · 7 sources 7 sources
- 38 What percentage of vitamin C derivatives actually works? This is not one question — it's five. Sodium ascorbyl phosphate (SAP) has RCT evidence at 5% for acne; CIR safety data covers SAP up to 3% and magnesium ascorbyl phosphate up to 3% as antioxidants; ascorbyl glucoside and 3-O-ethyl ascorbic acid have their own separate stability profiles at different doses. Pure L-ascorbic acid's 8-20% saturation curve does not apply to any of these — they are different molecules. Concentration guide · 4 sections · 8 sources 8 sources
- 35 What percentage of vitamin E (tocopherol) actually works? 1% alpha-tocopherol is the only concentration with controlled photoprotection data — every Pinnell-lab study used exactly 1%. No standalone dose-finding study exists, and CIR safety data covers a huge range (up to 36%) that has nothing to do with efficacy. Concentration guide · 4 sections · 5 sources 5 sources
- 40 What percentage of zinc oxide actually works? 25% is the FDA-permitted ceiling, but disclosed sunscreens run 5-10% zinc oxide in practice. Because zinc oxide is one of only two US actives that cover true long-UVA, higher zinc percentages generally mean more reliable broad-spectrum coverage — at the cost of a heavier white cast. Concentration guide · 3 sections · 6 sources 6 sources