1. Formulation Chemistry: What Is The Ordinary Hyaluronic Acid?
The Ordinary Hyaluronic Acid 2% + B5 is a water-based humectant serum combining multi-molecular weight hyaluronic acid and provitamin B5 to hydrate stratum corneum layers. It draws water into cutaneous tissue without relying on heavy lipid carriers.
Hyaluronic acid (HA) is a naturally occurring glycosaminoglycan found throughout human connective tissue and the extracellular matrix of the dermis. In topical formulation, however, raw hyaluronic acid possesses a molecular weight exceeding 1,000 to 1,800 kilodaltons (kDa). A macromolecule of this scale cannot cross an intact stratum corneum, whose intercellular lipid pathways generally limit passive diffusion to lipophilic molecules smaller than 500 Daltons (the 500-Dalton rule). To bypass this physiological barrier, The Ordinary incorporates low-, medium-, and high-molecular-weight forms of sodium hyaluronate alongside an advanced sodium hyaluronate crosspolymer, yielding a consolidated 2% active concentration.
The inclusion of sodium hyaluronate crosspolymer is technically significant. By polymerizing sodium hyaluronate chains into a dynamic lattice, the crosspolymer exhibits an exceptional water-binding capacity—exceeding that of standard linear HA—while forming a breathable, non-occlusive viscoelastic film on the skin surface. Concurrently, provitamin B5 (panthenol) at an effective concentration of approximately 1% to 2% supports surface hydration and stimulates fibroblast proliferation, accelerating epidermal barrier recovery. The formulation is buffered to a physiological pH range of 6.00 to 7.50, mirroring the natural pH of neutral aqueous vehicles to minimize irritation while preventing the hydrolytic degradation of the polymer backbone.
In late 2023, the brand updated this classic formula to introduce three essential ceramides alongside a lighter, less tacky delivery vehicle. This adjustment directly addresses the primary limitation of classical standalone humectant serums: water attraction without sufficient barrier lipids to prevent evaporative moisture loss.
2. Biophysical Mechanism: Molecular Weights and Transepidermal Water Loss
Topical humectants do not generate moisture; they relocate it via osmotic and thermodynamic gradients. Understanding how different molecular weights perform within the epidermis explains why incorrect usage can paradoxically cause surface tightness and flaking.
The molecular division within The Ordinary’s formulation serves distinct histological compartments:
- High-Molecular-Weight HA (1,000–1,500+ kDa): Remains entirely on the surface of the stratum corneum. It acts as an ambient film-forming agent, capturing ambient water molecules and reducing immediate evaporative loss while giving an instant tactile smoothing effect.
- Medium-Molecular-Weight HA (100–300 kDa): Penetrates the superficial layers of the stratum corneum, supporting inter-corneocyte hydration and increasing tissue elasticity within upper epidermal structures.
- Low-Molecular-Weight HA (under 50 kDa): Diffuses deeper into the lower stratum corneum and upper granulosum. While structurally beneficial for hydration signaling, low-molecular-weight fragments can trigger mild, transient pro-inflammatory pathways if applied to an active, weeping barrier breach.
- Panthenol (Provitamin B5): An alcohol analog of pantothenic acid that rapidly penetrates the stratum corneum to stabilize cellular membrane fluidity and enhance natural lipid synthesis.
The thermodynamic risk of any topical humectant lies in ambient relative humidity (RH). When environmental RH drops below 40% to 50%, atmospheric water vapor is scarce. If a 2% hyaluronic acid solution is applied to dry skin in an arid environment without an occlusive seal, the humectant draws water along the steepest gradient available: from the viable epidermis and dermis upward into the stratum corneum. Without an occlusive lipid barrier to block vapor transfer, that water evaporates rapidly into the dry air via accelerated Transepidermal Water Loss (TEWL). Clinical hydration is only maintained when the humectant is paired with water on application and sealed with a lipid-rich moisturizer.
3. Layering Order, Routine Hierarchy, and Active Interactions
Because skincare layering depends on vehicle viscosity, solvent polarity, and chemical pH stability, applying The Ordinary Hyaluronic Acid in the wrong sequence can lead to poor absorption, active destabilization, or severe product pilling.
As a non-reactive, water-based serum, hyaluronic acid should be applied directly after cleansing (or following watery toners and essences) and before anhydrous formulations, emulsions, heavy creams, and non-polar facial oils. Because its polymers form a structural surface matrix as they dry, applying a thick cream or silicone-heavy primer beforehand will completely obstruct the humectant’s ability to bind water at the skin surface. Conversely, applying high-viscosity HA before low-pH liquid exfoliants will buffer the acid’s contact with the skin, rendering chemical exfoliants less effective.
Consider how hyaluronic acid interacts with other primary clinical actives:
- L-Ascorbic Acid (Vitamin C): Direct ascorbic acid relies on an acidic vehicle (pH 2.5–3.5) to remain un-ionized for skin absorption. If you combine pure L-ascorbic acid and hyaluronic acid (pH 6.0–7.5) simultaneously in your palm, the HA buffers the acid, elevating its pH and significantly reducing its transdermal delivery. Apply pure Vitamin C first, wait 5 to 10 minutes for absorption and stratum corneum re-equilibration, and then apply the hyaluronic serum.
- Direct Exfoliating Acids (Glycolic, Lactic, Salicylic Acids): Similar to Vitamin C, AHAs and BHAs require low pH environments. Always apply chemical peels or exfoliating solutions first. Layering hyaluronic acid on damp skin immediately after a strong acid can also accelerate the acid’s penetration, potentially causing chemical burns or irritation.
- Retinoids (Retinol, Retinaldehyde, Pure Tretinoin): Hyaluronic acid does not chemically interfere with retinoid receptors. However, applying HA first on damp skin increases retinoid penetration rates, which can spike retinoid dermatitis. To reduce irritation, dermatologists recommend the sandwich method: apply hyaluronic acid, allow it to dry down completely, apply the retinoid, and follow with an occlusive moisturizer.
- Niacinamide: A completely non-conflicting pairing. Niacinamide functions smoothly within the same pH window (5.0–7.0) and complements HA by stimulating endogenous ceramide production.
- Copper Peptides: Susceptible to degradation under low pH and oxidative stress. The neutral pH of The Ordinary Hyaluronic Acid makes it an ideal hydration partner for peptide serums.
If you are coordinating multiple targeted serums in a single routine, using Skin Scan Genius allows you to evaluate your full product stack, verifying vehicle compatibility, identifying cumulative drying agents, and preventing polymer pilling before applying them to your face.
4. Clinical Application Protocol: Preventing Pilling and Dehydration
Pilling—the formation of rubbery, rolled-up product flakes upon touch—is the most common cosmetic issue reported with high-concentration hyaluronic acid formulations. This occurs due to the rapid evaporation of water from high-molecular-weight sodium hyaluronate cross-polymers, which leaves behind a fragile polymeric film that shears off under frictional mechanical stress.
To avoid pilling while maximizing skin hydration, follow this clinical application protocol:
- Ensure a Damp Surface: Never apply concentrated humectants to bone-dry skin. Following cleansing, leave a light veil of water on the face, or mist with a thermal water spray. This provides an external water reservoir for the HA molecules to bind, preventing them from pulling hydration out of the lower epidermis.
- Limit Dosing: Two to three drops are sufficient for the entire face. Dispensing a full pipette introduces excess polymer mass that cannot integrate into the upper stratum corneum, guaranteeing pilling once dried.
- Press, Do Not Rub: Spread the drops quickly across the skin and press gently with open palms. Vigorous rubbing shears the polymer chains, accelerating structural breakdown and foaming on the skin’s surface.
- Lock with an Occlusive or Emollient Barrier: Apply your lipid-based moisturizer while the hyaluronic acid layer is still slightly tacky (within 30 to 60 seconds). Do not allow the serum to dry completely into a tight, brittle film. The secondary moisturizer should contain occlusives (such as squalane, petrolatum, dimethicone, or plant butters) to physically seal the bound water into the stratum corneum.
If you wear daytime sunscreen or silicone-based foundation, allow the moisturized layer to set for two to three minutes. This prevents mechanical lifting of the underlying polymers during cosmetic application.
5. Skin Type Suitability, Barrier Status, and Clinical Precautions
While hyaluronic acid is labeled as universally compatible, its actual performance depends heavily on the physiological state of your epidermal barrier and your baseline sebum production.
For oily and acne-prone skin, the serum provides effective, weightless hydration. Oily skin frequently suffers from dehydration (a lack of water, not oil), often caused by harsh surfactants, alcohol-based toners, or drying anti-acne topicals like benzoyl peroxide. Because The Ordinary Hyaluronic Acid 2% + B5 is water-based and free of comedogenic esters, it hydrates without congesting the follicular infundibulum or promoting Cutibacterium acnes proliferation.
For dry (alipidic) skin, the product cannot function as a standalone hydrator. Alipidic skin lacks the structural intercellular lipids—specifically ceramides, cholesterol, and free fatty acids in their optimal 3:1:1 physiological ratio—needed to retain bound moisture. Using this serum without a substantial lipid cream will generally increase feelings of dermal tightness within hours of application.
Individuals managing chronic inflammatory conditions like rosacea, atopic dermatitis (eczema), or facial psoriasis should proceed with caution. While panthenol soothes inflammation, fragmented low-molecular-weight hyaluronic acid can stimulate inflammatory signaling cascades (via Toll-like receptors TLR2 and TLR4) when applied to an open, severely compromised stratum corneum. If you are experiencing an active inflammatory flare, skip multi-weight HA serums and consult a board-certified dermatologist for a barrier-repair regimen focused on physiological lipids and anti-inflammatory agents.
When introducing this serum into an existing multi-step routine, running your products through Skin Scan Genius can confirm whether your combination of active ingredients, preservatives, and penetration enhancers supports barrier health or risks compounding low-grade irritation.
Check it against your own routine. Skin Scan Genius reads a product’s full INCI list and flags conflicts with the actives you already use, by skin type and concern.
Frequently asked questions
Should I apply The Ordinary Hyaluronic Acid to wet or dry skin?
Always apply it to damp skin. Hyaluronic acid is an avid humectant that requires water to bind; applying it to dry skin—especially in low-humidity environments—can cause the molecules to draw moisture from deeper epidermal layers, increasing transepidermal water loss.
Why does The Ordinary Hyaluronic Acid pill under my moisturizer?
Pilling occurs when high-molecular-weight polymers dry into a film and shear under friction. To prevent this, use only 2 to 3 drops, pat the serum onto damp skin rather than rubbing, and apply your moisturizer immediately while the serum is still tacky.
Can I use The Ordinary Hyaluronic Acid with retinol and Vitamin C?
Yes, but stagger your application order. Apply low-pH actives like pure L-Ascorbic Acid or chemical exfoliants first and wait a few minutes before applying hyaluronic acid. For retinoids, you can apply hyaluronic acid first to buffer the skin, but ensure it is fully dry before applying the retinoid to avoid driving the active too deep too quickly.
Can hyaluronic acid break out acne-prone skin?
Hyaluronic acid itself is non-comedogenic and does not clog pores. However, if breakouts occur, they are typically caused by an unsealed formulation drying out the surface barrier, or by pairing the serum with an overly heavy, comedogenic occlusive cream layered on top.
