What concentration of THD-C should I start with?

Start with 10%, especially if you have sensitive skin or have not used vitamin C before. After 2-4 weeks, you can increase to 20% if desired. Reserve 30% for experienced users targeting significant photoaging.

Tetrahexyldecyl Ascorbate (THD-C) Vitamin C

Q: Should I use THD-C morning or night?

Either works well. THD-C is photostable so it is safe for daytime use with sunscreen. Some people prefer morning for antioxidant protection, while others prefer evening to pair with other treatments. Using it twice daily is also an option, particularly with lower concentrations.

Quick Reference

INCI Name: Tetrahexyldecyl Ascorbate

Common Names: THD Ascorbate, THDA, THD-C, oil-soluble vitamin C, lipid-soluble vitamin C

Category: Antioxidant, vitamin C derivative

Source: Synthetic (derived from L-ascorbic acid)

Nubeean Concentrations: 5%, 10%, 20%, 30%

Carrier System: Olive squalane

Skin Types: All skin types, particularly suited to sensitive skin, mature skin and those who cannot tolerate L-ascorbic acid

Cautions: Higher concentrations (20-30%) may cause initial redness or peeling, particularly in sun-damaged areas. Patch test first with higher strengths.

What It Is

Tetrahexyldecyl ascorbate is an oil-soluble form of vitamin C that was developed to overcome the significant limitations of traditional L-ascorbic acid. Where L-ascorbic acid is water-soluble, unstable and requires a very acidic pH (below 3.5) to penetrate skin - often causing irritation - THD-C is stable at neutral pH, easily penetrates the skin's lipid barrier and converts to active L-ascorbic acid once inside the skin cells.

The molecule works because of its esterified structure: four fatty chains attached to the ascorbic acid core make it lipophilic (fat-loving), allowing it to slip through the stratum corneum - the skin's waxy outer barrier - with ease. Research demonstrates that THD-C penetrates approximately three times more effectively than L-ascorbic acid at the same concentration, reaching both the epidermis and dermis where it can exert its collagen-supporting effects.

This matters for two reasons. First, you get more vitamin C where it's needed. Second, you can use it comfortably every day without the stinging, redness and pH sensitivity that makes L-ascorbic acid difficult for many people to tolerate.

How It Works

Enzymatic conversion. THD-C is a pro-vitamin - it doesn't work as vitamin C directly. Once it penetrates the skin, cellular esterase enzymes cleave off the fatty chains, releasing active L-ascorbic acid inside the cells where it's needed. This intracellular delivery mechanism means the vitamin C arrives protected, avoiding the oxidation that plagues topical L-ascorbic acid formulas.

Collagen synthesis support. Vitamin C is an essential co-factor for the enzymes that produce and crosslink collagen. Without adequate vitamin C, collagen formation is impaired. THD-C supports this process by ensuring vitamin C reaches the dermal fibroblasts responsible for collagen production. Research shows THD-C can increase expression of collagen types I and III - the primary structural collagens in skin.

MMP inhibition. Matrix metalloproteinases (MMPs) are enzymes that break down collagen and other structural proteins. UV exposure and aging both upregulate MMP activity. THD-C has been shown to inhibit MMP-1 and MMP-7 activity, helping preserve existing collagen while supporting new production.

Antioxidant protection. Once converted to L-ascorbic acid, THD-C provides free radical scavenging activity, protecting cells from UV-induced oxidative damage. However, research indicates that THD-C alone is a relatively poor antioxidant on the skin surface - its strength lies in delivering vitamin C inside cells rather than surface-level protection.

Melanin regulation. Vitamin C inhibits tyrosinase, the enzyme responsible for melanin production. By reducing melanin synthesis, THD-C can help fade existing hyperpigmentation and prevent new dark spots from forming. Clinical studies have demonstrated significant improvement in skin tone evenness with regular use.

Benefits

Primary Benefits

THD-C supports collagen production by ensuring vitamin C reaches the dermal cells where collagen synthesis occurs. Over time, this can improve skin firmness and reduce the appearance of fine lines. The ingredient also provides meaningful brightening effects through tyrosinase inhibition, helping to even out skin tone and fade hyperpigmentation.

Secondary Benefits

Because THD-C is stable and works at neutral pH, it's far gentler than L-ascorbic acid - making it accessible to sensitive skin types who previously couldn't tolerate vitamin C. The oil-soluble nature also means it integrates beautifully into anhydrous formulations, acting as both an active ingredient and part of the moisturising oil phase. Unlike water-soluble vitamin C that evaporates off the skin, THD-C in a squalane base provides lasting hydration.

Skin Concerns Addressed

THD-C addresses visible photoaging including fine lines, uneven texture and loss of firmness. It's effective for hyperpigmentation from sun damage, post-inflammatory marks and melasma (though melasma requires comprehensive management). It can help improve the appearance of acne scars through its collagen-supporting mechanism.

Formulation Matters

The choice of carrier oil significantly affects THD-C performance. We use olive squalane for several reasons.

Penetration support. Squalane mimics human sebum (it comprises about 13% of our skin's natural oils), which means it's readily accepted by the skin and enhances the penetration of anything dissolved in it. THD-C in squalane reaches deeper skin layers than THD-C in heavier oils.

Stability synergy. THD-C can degrade when exposed to singlet oxygen - a reactive form of oxygen present on skin from UV exposure. Research shows that combining THD-C with antioxidants like acetyl zingerone prevents this degradation. Our AM spray (Hydrate + Protect | Skin Shield) contains acetyl zingerone specifically to complement THD-C used afterward.

Multi-tasking formulation. Because squalane is an excellent moisturiser in its own right, our THD-C products serve double duty - you get your vitamin C treatment and your facial oil in one step. This simplifies routines and reduces the number of products needed.

Who Benefits Most

Sensitive skin. If you've tried L-ascorbic acid serums and experienced stinging, redness or irritation, THD-C offers the benefits of vitamin C without the harsh pH requirements. Our 10% Sensitive C Serum is specifically designed for reactive skin types.

Mature and sun-damaged skin. The combination of collagen support, MMP inhibition and brightening makes THD-C particularly effective for addressing the cumulative effects of UV exposure. Higher concentrations (20-30%) may trigger a beneficial renewal response in significantly photodamaged areas.

Hyperpigmentation concerns. Whether from sun damage, hormonal changes or post-inflammatory marks, the tyrosinase-inhibiting action of THD-C can help fade existing pigmentation while preventing new spots.

Those who want simplicity. If you prefer a streamlined routine, a THD-C serum in squalane replaces both your vitamin C treatment and your facial oil.

Fitzpatrick skin types I-II with sun exposure history. Fair skin that has accumulated UV damage often needs cell turnover support but can't tolerate aggressive treatments. THD-C provides a gentler approach to skin renewal.

How We Use It

Products Containing Tetrahexyldecyl Ascorbate

Sensitive C Serum Concentration: 10% THD-C Key Ingredients: Olive squalane, tetrahexyldecyl ascorbate, mixed tocopherols Use: Fragrance-free daily vitamin C for sensitive skin, suitable for morning or evening

Sensitive C Serum PLUS Concentration: 10% THD-C + 0.5% bakuchiol Key Ingredients: Olive squalane, tetrahexyldecyl ascorbate, bakuchiol, mixed tocopherols Use: Enhanced version with bakuchiol for additional retinol-like benefits, fragrance-free

Vitamin C 20% High Strength Serum Concentration: 20% THD-C Key Ingredients: Olive squalane, tetrahexyldecyl ascorbate, mixed tocopherols, steam distilled lemon oil (fragrance-free option available) Use: Higher strength for visible signs of aging and pigmentation

Vitamin C 20% + 1% Bakuchiol High Strength Serum Concentration: 20% THD-C + 1% bakuchiol Key Ingredients: Olive squalane, tetrahexyldecyl ascorbate, bakuchiol, mixed tocopherols, steam distilled lemon oil (fragrance-free option available) Use: Comprehensive anti-aging with combined vitamin C and retinol-like benefits

Vitamin C 30% Professional Strength Concentration: 30% THD-C Key Ingredients: Olive squalane, tetrahexyldecyl ascorbate, mixed tocopherols, steam distilled lemon oil (fragrance-free option available) Use: Therapeutic strength for experienced users, significant photoaging or hyperpigmentation. Can be used 2-3 times weekly rather than daily.

Vitamin C 30% + 1% Bakuchiol Professional Strength Concentration: 30% THD-C + 1% bakuchiol Key Ingredients: Olive squalane, tetrahexyldecyl ascorbate, bakuchiol, mixed tocopherols, steam distilled lemon oil (fragrance-free option available) Use: Maximum strength combination for experienced users targeting significant photoaging

Blue Tansy Balm Concentration: 5% THD-C Key Ingredients: Olive squalane, blueberry seed oil, organic jojoba oil, jojoba esters, beeswax, tetrahexyldecyl ascorbate, mixed tocopherols, blue tansy essential oil Use: Nourishing balm with gentle vitamin C for those who prefer a richer texture

Using Our THD-C Products

THD-C serums are designed to be the final step in your skincare routine - the oil layer that seals in everything applied before it. Apply to clean skin after any water-based serums (like our Hydrate + Protect | Skin Shield or copper peptide serums) have absorbed.

For morning use, follow with sunscreen. While THD-C is photostable (unlike L-ascorbic acid), protecting your skin from UV damage is always important when targeting photoaging.

For evening use, THD-C can be applied over copper peptide serum as the final moisturising step. Despite historical caution about combining vitamin C and copper peptides, THD-C's oil-soluble nature means it occupies a completely different phase than water-based peptide serums - they don't meaningfully interact on the skin.

A Note on Fragrance

Our Sensitive C Serum and Sensitive C Serum PLUS are completely fragrance-free. The only scent is the faint, natural aroma of olive squalane.

The 20% and 30% strength serums are available with or without steam distilled lemon oil. The lemon oil was added at customer request - many people find that a fresh, citrus scent creates a more enjoyable daily ritual and helps psychologically reinforce the "vitamin C" benefits. Steam distilled lemon oil does not contain the furanocoumarins that cause photosensitivity in cold-pressed citrus oils, so it is safe for daytime use. However, if you prefer completely fragrance-free skincare, all higher-strength serums are available without lemon oil.

The Formulation Story

THD-C dissolved in squalane is deceptively simple - just two or three ingredients depending on the product. But that simplicity is intentional.

We started making THD-C serums because customers who loved what vitamin C could do for their skin couldn't tolerate L-ascorbic acid formulas. The stinging, the low pH, the instability - it was too much for many people. THD-C solved those problems elegantly.

The 30% strength came about differently. A customer asked for a high-dose formulation they could use two or three times weekly instead of daily - a therapeutic approach rather than maintenance. We made it, they used it, and the feedback was remarkable. People with significant sun damage reported not just gradual improvement, but an initial period of redness followed by what looked and felt like a mild peel. In areas of known sun damage, some experienced tiny papules that would persist for days before clearing, leaving smoother skin behind.

This wasn't what we expected. But it aligns with research showing that high-concentration THD-C activates type I interferon signalling - part of the innate immune system involved in recognising and responding to abnormal cells. At 30%, you may be triggering a targeted response in photodamaged tissue.

We now guide customers to use 30% as a therapeutic treatment (2-3 times weekly) rather than daily, and we're always honest about what to expect during the initial period.

Usage Guidelines

Starting Out

If you're new to THD-C, begin with the 10% Sensitive C Serum used once daily. This allows your skin to adjust to the ingredient without overwhelming it.

Building Up

After 4-12 weeks at 10%, you can increase to 20% if desired. Most people find 20% provides excellent results for visible signs of aging and pigmentation without needing higher concentrations.

The 30% Approach

30% THD-C is a therapeutic strength, not a daily maintenance dose. Use 2-3 times weekly, allowing rest days between applications. Expect some initial redness and potentially light peeling, particularly in sun-damaged areas. This typically subsides after the first few weeks as skin adjusts.

Application

Apply 3-4 drops to clean, dry skin. Gently press and smooth over face and neck. Follow with sunscreen in the morning. No additional moisturiser is typically needed as the squalane provides sufficient hydration for most skin types.

Expected Timeframe

Brightening effects may be visible within 2-4 weeks. Improvements in fine lines and skin texture typically become noticeable around 6-8 weeks. Significant collagen remodelling takes 3-6 months of consistent use.

Contraindications and Cautions

Patch test first, especially with 20% and 30% strengths. Apply a small amount to your inner arm and wait 24 hours before using on your face.

Initial adjustment period. Higher concentrations may cause temporary redness or mild peeling, particularly in sun-damaged areas. This usually subsides within the first few weeks. If severe irritation occurs, reduce frequency or switch to a lower concentration.

Sun protection. While THD-C doesn't cause photosensitivity, if you're treating photoaging you should be protecting your skin from further UV damage. Use sunscreen daily.

Pregnancy and breastfeeding. THD-C is generally considered safe during pregnancy (unlike retinoids), but consult your healthcare provider if you have concerns.

Avoid mixing with: L-ascorbic acid products (redundant and may cause irritation), very low pH products applied immediately before or after.

Research and Evidence

Clinical studies have demonstrated that 30% THD-C formulations improve signs of photoaging including pigmentation, fine lines and overall skin quality after 6 weeks of twice-daily use. Research also shows that THD-C combined with acetyl zingerone provides superior collagen production and antioxidant effects compared to either ingredient alone - this is why we recommend using our THD-C serums with our Hydrate + Protect | Skin Shield spray which contains acetyl zingerone.

The penetration advantage of THD-C over L-ascorbic acid has been confirmed in membrane studies, showing approximately threefold greater penetration at equivalent concentrations.

Frequently Asked Questions

What is the difference between THD-C and regular vitamin C (L-ascorbic acid)? THD-C is oil-soluble and stable, while L-ascorbic acid is water-soluble, unstable and requires a very low pH to work. THD-C penetrates skin more effectively, converts to active vitamin C inside cells, and is far gentler - making it suitable for sensitive skin types who can't tolerate traditional vitamin C serums.

Can I use THD-C with copper peptides? Yes. THD-C is oil-based while copper peptides are water-based - they occupy different phases and don't meaningfully interact on skin. You can apply copper peptide serum first, let it absorb, then follow with THD-C as your final oil step.

Should I use THD-C morning or night? Either works well. THD-C is photostable so it's safe for daytime use (always with sunscreen). Some people prefer morning for the antioxidant protection, while others prefer evening to pair with other treatments. Using it twice daily is also an option, particularly with lower concentrations.

What concentration should I start with? Start with 10%, especially if you have sensitive skin or haven't used vitamin C before. After 4-12 weeks, you can increase to 20% if desired. Reserve 30% for experienced users targeting significant photoaging.

Why does 30% THD-C cause a reaction on my skin? High-concentration THD-C can activate type I interferon signalling - part of your immune system's response to abnormal cells. In sun-damaged areas, this may manifest as temporary redness, mild peeling or small papules that resolve after a few days. This typically only occurs during the initial adjustment period and leaves improved skin afterward.

How is this different from bakuchiol products? THD-C is vitamin C; bakuchiol has retinol-like effects. They work through different mechanisms and can be used together. Our combination serums (Sensitive C PLUS and the +Bakuchiol versions) provide both benefits in one product.

Shop Vitamin C Products → Vitamin C Serum Collection

Australian made | Small batch | Clean formulation | Fragrance-free