Skincare formulation questions, answered

Ingredient compatibility

• Can avobenzone and octinoxate be used together?

  Avobenzone and octinoxate degrade each other under UV light.

  Source: Sayre RM et al. Photodermatol Photoimmunol Photomed. 2005;21(2):84-92

• Do copper peptides affect vitamin C?

  Copper ions catalyse the oxidation of ascorbic acid.

  Source: Buettner GR. Free Radic Res Commun. 1986;1(6):349-53

• Can I use benzoyl peroxide with retinol?

  Benzoyl peroxide oxidises tretinoin and retinol when mixed in the same layer.

  Source: Martin B et al. Br J Dermatol. 1998;139 Suppl 52:8-11

Concentration

• What percentage of niacinamide is best?

  Most measured niacinamide benefit occurs between 2% and 5%.

  Source: Bissett DL et al. Dermatol Surg. 2005;31(7 Pt 2):860-5

• Is fragrance in skincare a problem?

  Fragrance is the most common cause of cosmetic contact allergy.

  Source: de Groot AC, Frosch PJ. Contact Dermatitis. 1997;36(2):57-86

• What does urea do in skincare?

  Urea hydrates at low concentrations and becomes keratolytic near 10%.

  Source: Celleno L. Dermatol Ther. 2018;31(6):e12690

• Does lactic acid increase sun sensitivity?

  Higher-strength lactic acid exfoliates more aggressively and increases sun sensitivity.

  Source: Smith WP. Int J Cosmet Sci. 1996;18(2):75-83

Ingredient context

• Do niacinamide and vitamin C cancel each other out?

  Niacinamide and vitamin C do not cancel out at cosmetic temperatures.

  Source: Wohlrab J, Kreft D. Skin Pharmacol Physiol. 2014;27(6):311-15

• Why are limonene and linalool listed separately?

  EU rules require 26 fragrance allergens to be named individually on the label.

  Source: EU Cosmetics Regulation 1223/2009, Annex III

• Which ingredients feed fungal acne?

  Malassezia metabolises most fatty acids and their esters.

  Source: Ashbee HR, Evans EG. Clin Microbiol Rev. 2002;15(1):21-57

• Do ceramides work better with other ingredients?

  Ceramides repair the barrier best alongside cholesterol and free fatty acids.

  Source: Man MQ et al. Arch Dermatol. 1993;129(6):728-38

• Is zinc oxide sunscreen better?

  Zinc oxide is photostable and covers both UVA and UVB.

  Source: Smijs TG, Pavel S. Nanotechnol Sci Appl. 2011;4:95-112

• Why is azelaic acid so thick and grainy?

  Azelaic acid dissolves poorly, so formulas suspend it rather than dissolving it.

  Source: Fitton A, Goa KL. Drugs. 1991;41(5):780-98

• Are vitamin C derivatives as good as ascorbic acid?

  Vitamin C derivatives are more stable but must convert in skin to act.

  Source: Stamford NP. J Cosmet Dermatol. 2012;11(4):310-17

• What is DMDM hydantoin in skincare?

  DMDM hydantoin preserves products by slowly releasing small amounts of formaldehyde.

  Source: de Groot AC et al. Contact Dermatitis. 2010;62(1):2-17

• Why is methylisothiazolinone banned in some products?

  The EU banned methylisothiazolinone from leave-on cosmetics after a rise in contact allergy.

  Source: SCCS/1521/13; EU Regulation 2017/1224

• What does propylene glycol do in skincare?

  Propylene glycol increases how much of the other ingredients the skin absorbs.

  Source: Fasano WJ et al. Toxicol In Vitro. 2011;25(8):1664-70

• Does centella asiatica actually work?

  Centella asiatica has clinical support for barrier recovery and reducing visible redness.

  Source: Bylka W et al. Adv Clin Exp Med. 2013;22(3):421-8

pH and efficacy

• Why is my vitamin C serum not working?

  L-ascorbic acid requires a pH below 3.5 to penetrate skin effectively.

  Source: Pinnell SR et al. Dermatol Surg. 2001;27(2):137-42

• What pH does salicylic acid need to exfoliate?

  Salicylic acid only exfoliates in its free-acid form, below roughly pH 4.

  Source: Kornhauser A et al. Clin Cosmet Investig Dermatol. 2010;3:135-42

• Why do some glycolic acid products feel gentler?

  Neutralising glycolic acid lowers its free-acid content, which is the exfoliating fraction.

  Source: Green BA et al. Clin Dermatol. 2009;27(5):495-501

Stability

• Why does my retinol stop working over time?

  Retinol degrades in the presence of water, light and oxygen.

  Source: Temova Rakuša Ž et al. Pharmaceutics. 2021;13(10):1738

• What makes a retinol formula stable?

  Retinol keeps its potency longest in an anhydrous base with antioxidants.

  Source: Temova Rakuša Ž et al. Pharmaceutics. 2021;13(10):1738

• What is the best vitamin C combination?

  Ferulic acid and vitamin E stabilise L-ascorbic acid and raise its photoprotection.

  Source: Lin FH et al. J Invest Dermatol. 2005;125(4):826-32

• Does avobenzone sunscreen stop working in the sun?

  Avobenzone loses much of its UVA protection within an hour without a photostabiliser.

  Source: Chatelain E, Gabard B. Photochem Photobiol. 2001;74(3):401-6

• Can adapalene be used with benzoyl peroxide?

  Adapalene is photostable and does not degrade in contact with benzoyl peroxide.

  Source: Martin B et al. Br J Dermatol. 1998;139 Suppl 52:8-11

Formulation base

• Why does hyaluronic acid make my skin drier?

  Hyaluronic acid draws in water but does not stop it evaporating.

  Source: Draelos ZD. Dermatol Ther. 2012;25(3):222-5

Contact allergens

Families from the European baseline and NACDG patch-test series, with documented cross-reactivity.

• Can vitamin E cause a skin reaction?

  Tocopherol is an uncommon but documented contact allergen in leave-on products.

  Source: Kosari P et al. Dermatitis. 2010;21(3):148-53

• What is colophony in cosmetics?

  Colophony is pine resin, and it shares oxidised terpene chemistry with several fragrance materials.

  Source: Downs AM, Sansom JE. Contact Dermatitis. 1999;41(5):305

• Which plants are in the Compositae allergy family?

  Chamomile, arnica, calendula and yarrow share the sesquiterpene lactones behind Compositae allergy.

  Source: Paulsen E. Contact Dermatitis. 2017;76(1):1-10

• Can I use propolis if allergic to Balsam of Peru?

  Propolis overlaps in composition with Balsam of Peru and the two frequently co-react.

  Source: Walgrave SE et al. Dermatitis. 2005;16(4):209-15

• Why does old tea tree oil cause reactions?

  Tea tree oil sensitisation is driven by oxidation products that form as the oil ages.

  Source: Rutherford T et al. Australas J Dermatol. 2007;48(2):83-7

• What is PPD in hair dye?

  Para-phenylenediamine is the principal permanent hair-dye allergen.

  Source: European baseline series; Thyssen JP et al. Contact Dermatitis. 2008;59(6):327-43

• Why does lanolin irritate some people?

  Lanolin reactions occur more frequently on already-damaged or broken skin.

  Source: European baseline series; Fransen M et al. Contact Dermatitis. 2018;78(4):237-44

• What should I avoid if allergic to Balsam of Peru?

  Balsam of Peru contains cinnamates and benzoates, which co-react with many fragrance components.

  Source: Api AM. Food Chem Toxicol. 2006;44(12):2056-8

• What is Fragrance Mix I in a patch test?

  Fragrance Mix I screens eight components that detect most fragrance allergy.

  Source: European baseline series; de Groot AC, Frosch PJ. Contact Dermatitis. 1997;36(2):57-86

• What does Fragrance Mix II test for?

  Fragrance Mix II covers six fragrance allergens that Mix I does not detect.

  Source: Frosch PJ et al. Contact Dermatitis. 2005;52(4):207-15

• Why do nickel and cobalt allergy occur together?

  Cobalt and nickel occur together in alloys and pigments, so sensitivity to both is common.

  Source: European baseline series; Thyssen JP et al. Contact Dermatitis. 2011;65(3):125-37

• Is there nickel in cosmetics?

  Nickel appears in cosmetics as a trace contaminant of some mineral pigments.

  Source: European baseline series; Ahlström MG et al. Contact Dermatitis. 2019;81(4):227-41

• Which skincare preservatives release formaldehyde?

  Quaternium-15, DMDM hydantoin, imidazolidinyl and diazolidinyl urea all release formaldehyde.

  Source: de Groot AC et al. Contact Dermatitis. 2010;62(1):2-17

• Why was methylisothiazolinone banned in leave-on products?

  The EU banned methylisothiazolinone from leave-on cosmetics after a documented rise in contact allergy.

  Source: SCCS/1521/13; Urwin R et al. Contact Dermatitis. 2017;76(1):37-40

• Are parabens a common cause of skin allergy?

  Parabens have a low rate of contact allergy despite their public reputation.

  Source: European baseline series; Cashman AL, Warshaw EM. Dermatitis. 2005;16(2):57-66

• Is propylene glycol an allergen or an irritant?

  Propylene glycol causes both true allergic reactions and simple dose-dependent irritation.

  Source: Jacob SE, Scheman A, McGowan MA. Dermatitis. 2018;29(1):3-5

• What causes cocamidopropyl betaine allergy?

  Cocamidopropyl betaine allergy is usually caused by amidoamine manufacturing residues.

  Source: Foti C et al. Contact Dermatitis. 2010;63(2):109-10

• Which sunscreen filters cause photoallergy?

  Benzophenone filters are among the most frequent causes of photoallergic contact dermatitis.

  Source: Heurung AR et al. Dermatitis. 2014;25(6):289-326