Vitamin C in Skincare: L-Ascorbic Acid vs Tetrahexyldecyl Ascorbate
Skincare Science · 2026
Vitamin C in Skincare: L-Ascorbic Acid vs Tetrahexyldecyl Ascorbate
Vitamin C is one of the most-studied antioxidants in skincare — but the form on the ingredient list determines stability, penetration and tolerability. Here is the science behind the C-Tetra family.
Why topical vitamin C is studied so much
Topical vitamin C (ascorbic acid) is recognised as a primary cutaneous antioxidant. It contributes to collagen synthesis as a cofactor for the prolyl and lysyl hydroxylase enzymes, helps regenerate vitamin E, and quenches free radicals from UV exposure. The challenge is that vitamin C is notoriously unstable — it oxidises rapidly when exposed to air, light or water, which can lead to a loss of activity over time.
L-ascorbic acid: the original form
L-ascorbic acid (L-AA) is the bio-active form of vitamin C. It is the most direct way to deliver vitamin C topically, but has trade-offs:
- Water-soluble — needs an acidic pH (usually below 3.5) to penetrate the skin barrier
- Highly unstable — oxidises to dehydroascorbic acid then further to inactive products on contact with air
- Often poorly tolerated by sensitive skin at the concentrations needed for activity
Lipid-soluble derivatives: THD ascorbate
Tetrahexyldecyl ascorbate (THD-C, also known as ascorbyl tetraisopalmitate or ATIP) is an esterified, lipid-soluble vitamin C derivative. Because it is oil-soluble, it is more compatible with the lipid-rich stratum corneum and remains stable in anhydrous formulations without needing low pH.
An open-label clinical study published in 2024 in the Journal of Cosmetic Dermatology reported that a topical serum containing tetrahexyldecyl ascorbate plus acetyl zingerone improved the appearance of photodamaged skin over 12 weeks of use.[1] Independent industry research has previously shown topical vitamin C combinations with vitamin E and a peptide can reduce visible facial-ageing signs.[2]
Medik8®'s C-Tetra family
Medik8® C-Tetra® serums use stabilised tetrahexyldecyl ascorbate as the primary vitamin C active. The line spans different concentrations and formats:
- C-Tetra® — entry-strength serum suited to first-time vitamin C users and sensitive skin
- C-Tetra® Cream — moisturising vitamin C cream
- C-Tetra® Luxe — higher concentration THD-C with vitamin E
- C-Tetra® Advanced — newer launch with additional antioxidant support
- Super C Ferulic® — multi-form vitamin C with ferulic acid
- Daily Radiance Vitamin C® — vitamin C SPF 30 day cream
How to use vitamin C in your routine
- Morning, after cleansing and toning. Vitamin C pairs naturally with sunscreen — together they provide antioxidant + UV protection.
- 4–5 drops across face, neck and décolletage; let absorb before next step.
- Store away from heat and direct sunlight. While stabilised forms are more robust than L-AA, sensible storage extends shelf life.
- Always follow with broad-spectrum SPF in the morning.
Featured Medik8® vitamin C
Shop Medik8® vitamin C in NZ
Authentic, fresh, and authorised — only from Verdo Cosmetics.
Shop the C-Tetra® range →References
- Min M et al. (2024). Open-label topical application of tetrahexyldecyl ascorbate and acetyl zingerone containing serum improves the appearance of photodamaged skin. J Cosmet Dermatol. View
- Lintner K (2020). A serum containing vitamins C & E and a matrix-repair tripeptide reduces facial signs of aging as evidenced by Primos analysis. J Cosmet Dermatol. View
