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In beauty speak, there’s a dedicated term for the unpleasant effects of retinol use: the retinol uglies. Why there isn’t one for the skin-ravaging aftermath of vitamin C that many of us with sensitive skin often experience is a mystery. Might I submit “ascorbic agitation” for consideration?
But it doesn’t have to be this way! We don’t have to suffer through redness, irritation, and sensitivity just to use topical vitamin C. Turns out, there’s a gentler alternative to L-ascorbic acid (LAA), the pure yet caustic form of vitamin C that we’ve been using in our skin care products for decades. This form penetrates more easily into the skin’s deeper layers than LAA while being gentler. Meet tetrahexyldecyl ascorbate (aka THDA or THDC) who really is that girl. She’s not exactly a new ingredient, but has been popping up more recently as of late, perhaps in response to just how many folks self-identify as having “sensitive skin” these days.
How Vitamin C Benefits Skin
Before we get into THDA’s benefits, it’s worth reiterating why topical vitamin C is an active ingredient beloved by skin experts of all classifications. Like retinol, vitamin C is a multi-tasking powerhouse, a Swiss Army knife of skin care. “It improves the appearance of lines and wrinkles and helps with skin firmness, hyperpigmentation, and textural changes,” says Loretta Ciraldo, MD, a board-certified dermatologist in Miami. “And it prevents major cell damage after UV exposure, so it’s a great active to prevent photoaging.” (Photoaging is when the skin ages prematurely due to sun exposure, resulting in free radical damage.)
“Free radicals are highly reactive atoms and molecules that steal electrons from other molecules. They can overwhelm the skin’s natural antioxidant defenses, attacking and degrading collagen and elastin, which are the proteins crucial to skin’s firmness and elasticity,” adds Kelly Dobos, a cosmetic chemist and adjunct professor of cosmetic science at the University of Cincinnati. Free radicals also come at us via smoke and pollution, so a potent antioxidant like vitamin C is crucial in safeguarding our skin’s defenses.
In addition to preventing environmental damage and building up collagen and elastin, vitamin C is valuable for evening out hyperpigmentation for brighter-looking skin. “Vitamin C plays a critical role in reducing pigmentation by inhibiting an enzyme called tyrosinase, which is essential in melanin production,” says Jenny Liu, MD, a board-certified dermatologist in Minneapolis and assistant professor at the University of Minnesota. “By blocking tyrosinase activity, vitamin C helps lighten existing dark spots and prevents new ones from forming."
And the ingredient has receipts. “Dermatologists love vitamin C because of the volume of research that supports its benefits for the skin,” adds Rosemarie Ingleton, MD, a board-certified dermatologist based in New York. One study, for instance, points to skin appearing smoother, less wrinkled, and less pigmented when topical vitamin C was applied to skin with photodamage and melasma. “As long as your skin is not sensitive to it, it is helpful for practically everyone because it offers antioxidant protection,” says Dr. Ingleton.
The Problem With Traditional Vitamin C
That’s where things get complicated, because a lot of people can become sensitized by products containing L-ascorbic acid (LAA). That’s because LAA is not easily able to penetrate the skin in and of itself, says Krupa Koestline, a cosmetic chemist and the founder of KKT Innovation Labs. “In order to be effective, it has to be formulated at a pH lower than 3.5, which can be irritating, since the natural pH of our skin is between 4.7 and 5.75,” adds Dr. Ingleton.
Apart from getting spicy when applied to skin, LAA is also notoriously unstable as an ingredient, and degrades rapidly in the presence of light and air, making it difficult to keep it shelf-stable in skin care products for extended periods of time. “When developing cosmetic products we usually target a two-year shelf life, and ideally, we want them to be efficacious throughout that entire time period,” says Dobos. She says the degradation rate of LAA in a cosmetic depends on the formulation’s pH, packaging, and storage conditions, making it hard to give a definitive answer on how long it remains effective.
A telltale sign that your LAA product is past is prime is when it turns a dark-brown hue or begins to smell like hotdog water. Renée Rouleau, a celebrity esthetician and the founder of Renée Rouleau Professional Skin Care, says that in her experience, she’s noticed some LAA-based products turn a deep orange in a matter of days. That’s why entire brands—like Exponent—have been created around the idea of providing serums that dispense a metered dose of powdered LAA which needs to be freshly mixed into a liquid base, ensuring its freshness before every use.
It’s clear that LAA, in its traditional form, can be problematic, both for formulators and consumers. “LAA has been sold as the do-all of skin care. But out of a hundred LAA serums out there, there are probably two doing what they need to do,” says Koestline. “And it is pretty upsetting. It's not what people have sold it to be.”
A Better Way to Reap the Benefits of Vitamin C
When it comes to vitamin C in skin care, there are two categories: active and inactive. The only active form is LAA. All other forms of vitamin C are considered inactive because they have to be converted to LAA within the skin before they can get to work.
THDA is one of the many such inactive derivatives of vitamin C on the market that aims to overcome the deficiencies of LAA. Others include ascorbyl palmitate, ascorbyl glucoside, 3-O ethyl ascorbic acid, sodium ascorbyl phosphate, and magnesium ascorbyl phosphate. “I consider LAA to really be pure, while the others are substantially modified and structurally very different molecules,” says Dobos. While usefulness and efficacy vary amongst these derivatives, many of the experts we spoke to were clear that THDA came out on top.
As compared to LAA, THDA penetrates better and remains more stable—because it’s formulated to be that way. “LAA is inherently unstable and prone to oxidation because its structure readily reacts with oxygen in the atmosphere. THDA addresses this by chemically modifying ascorbic acid with a lipid-based tail during manufacturing, which stabilizes the molecule and makes it oil-soluble,” says Koestline. This modification not only improves THDA’s stability but also allows it to integrate smoothly into oil-based formulations.
And lastly, since THDA does not convert into active LAA until it comes into contact with the oils on the skin, it results in a gentler release of the active ingredient. “Unlike LAA, which is water-soluble and requires a low pH to remain stable, THDA is pH-independent. This makes it less irritating and suitable for sensitive skin, as it bypasses the need for an acidic environment, which can sometimes cause redness or stinging,” says Dr. Liu.
This allows THDA to be formulated in products with a skin-neutral or slightly acidic pH, which is less likely to disrupt the skin barrier or cause irritation. For example, Rouleau’s Vitamin C&E Treatment, which uses THDA, has a pH level of 5.3, as compared to some brands using LAA, which can have a pH of 3.5 or lower.
“When I think of the vitamin C derivatives, I would rank THDA the highest because of its combination of improved stability and ability to penetrate the skin more deeply,” says Dobos. “LAA's stability is so limiting. If I can work with something that is more stable while still providing valuable, meaningful benefits and can be formulated in a way that is more tolerable for more skin types, I’m going to choose that option.”
Why THDA Works Best for Sensitive Skin
Formulators like Dobos and Koestline appreciate THDA for not making them jump through hoops while building a product. Dermatologists love it because it allows even their most most sensitive patients the opportunity to reap the benefits of vitamin C. Dr. Ciraldo specifically selected THDA for her brand’s Anti-Aging Repair Serum because she wanted an active that would be a super-effective—yet super-gentle—agent at producing visible change. While one of her brand’s other products has LAA and is very effective on hyperpigmentation, it does cause “some skin changes that are similar to retinol in terms of peeling and redness,” she says. “LAA is a great active but never for sensitive, rosacea-prone skin.”
Dr. Ingleton’s concern was creating a safe vitamin C product for melanated skin types. “My aim was to allow melanated skin and sensitive skin individuals to tolerate using effective levels of vitamin C to address their pigmentary skin concerns without the increased risk of irritation that is usually associated with many vitamin C products,” she says. “Those with melanin-rich skin may benefit more from a vitamin C that doesn’t irritate their skin, since irritation can lead to hyperpigmentation.”
THDA also has research suggesting efficacy for improving different types of hyperpigmentation.
“Research indicates that THDA is effective in addressing various forms of hyperpigmentation, including sunspots, age spots, and post-inflammatory hyperpigmentation, because of its ability to penetrate the skin deeply and inhibit melanin production effectively,” says Dr. Liu. “Its stability and penetration also allow for sustained release and action, which is beneficial in managing pigment concerns like melasma.”
What Else You Should Know About THDA
As wonderful as THDA is, it’s not quite a silver bullet. While it does penetrate skin more deeply, it needs to be converted into LAA by the skin. And the math for that hasn’t been mathed out yet. “It should be an easy answer, but there isn't one. We know THDA penetrates the skin three times more effectively than LAA. But we don’t have the studies that tell us to use [X-amount of] THDA to make a certain amount of LAA,” explains Koestline.
Until science gets us there, follow a few golden rules from our pros to get the most of your THDA product. To determine if the product you’re considering is made with THDA, you’ll have to scan the ingredient list. “Products will usually list the specific type of Vitamin C on the ingredient list, though it may not always be prominent on the packaging,” says Dr. Liu. “THDA may appear as ‘tetrahexyldecyl ascorbate’ while LAA is listed as ‘ascorbic acid”
Also, packaging, in this case, is everything. Airless pump should be the sexiest marketing term for brands selling you any vitamin C product, and this applies to THDA ones as well. “Ignore all products in droppers or jars,” says Koestline. And because “formulations with THDA still require protection from heat, and UV light, so air-tight, opaque packing is important,” says Dobos.
And while THDA is more stable in comparison to LAA, it’s still able to degrade. A careful formulator will solve for that by adding additional antioxidants to the mix, so keep an eye out for those. “Vitamin C and its derivatives benefit by being formulated with other antioxidants like vitamin E, ferulic acid, and acetyl zingerone, an antioxidant derived from ginger,” says Dobos. Rouleau adds that she's found the ingredient is most effective in products that have extended contact with the skin. “THDA requires time to penetrate the skin, so it’s best used in leave-on products. I don’t recommend it in products that rinse off the skin because there simply is no time for THDA to get to work,” she says.
Keep scrolling to see shop products that put THDA center stage.
