Hyaluronic Acid: What It Does and What It Can't

Hyaluronic acid is everywhere. Serums, moisturisers, sheet masks, lip balms — even some sunscreens list it now, usually with a claim about “deep hydration” or “plumping from within.” It’s become the skincare ingredient that nobody questions, which is exactly when it’s worth questioning.

The reality is more interesting, and more limited, than the bottle copy. Hyaluronic acid is a genuinely useful hydrating ingredient with solid science behind it — but “solid science” and “does everything the label says” aren’t the same sentence.

What it actually is

Hyaluronic acid (HA) is a glycosaminoglycan — a long sugar chain — that your body already makes in large quantities. It sits in the spaces between cells in your skin, joints, and eyes, where its main job is holding onto water. A single gram of HA can bind up to six litres of water, which is an extraordinary capacity and the fact that launches a thousand marketing claims.

The HA in your skin is part of the extracellular matrix that keeps tissue hydrated, plump, and structurally supported. Your body produces less of it as you age — roughly 1% less per year from your mid-twenties — which is one reason skin gets drier and thinner over time. That biological context is real, and it’s what makes topical HA sound like an obvious fix: put back what time takes away.

The complication is that “putting it back” topically isn’t the same thing as replenishing what your skin has lost. That distinction matters more than most product pages let on.

The molecular weight question

This is where hyaluronic acid gets genuinely interesting, and where most skincare content gets lazy.

HA comes in different molecular weights, and they behave very differently on skin. High molecular weight HA (over 1,000 kDa) forms a film on the skin surface, drawing moisture from the air and reducing water loss. It’s too large to penetrate the epidermis — the 500 Dalton rule applies here, and high-weight HA is orders of magnitude above that threshold.

Low molecular weight HA (under 50 kDa) can penetrate deeper into the outer layers of skin. Some studies show it reaches the upper epidermis, where it may support hydration from within. But there’s a caveat that rarely makes the marketing: some research suggests that very low molecular weight HA fragments can trigger a mild inflammatory response. The skin reads small HA fragments as damage signals — not unlike how the body responds to wound debris. Whether that’s clinically meaningful in a well-formulated cosmetic product is still being debated, but it’s a more nuanced picture than “smaller = deeper = better.”

Most modern serums use a blend of molecular weights, which is probably the most sensible approach given what we know. You get the surface film from high-weight HA and some penetration from lower-weight fractions. Products that trumpet their “multi-weight” formulation are, for once, describing something that actually makes scientific sense rather than just marketing sense.

What the evidence actually supports

Hydration. This is HA’s strongest claim, and it’s well earned. Multiple studies confirm that topical HA improves skin hydration and reduces transepidermal water loss. The effect is real, measurable, and consistent across different formulations. If your skin is dehydrated — tight, dull, fine lines that appear when you’re not drinking enough water or your barrier is compromised — HA will help. It’s one of the most reliable humectants available.

Fine lines and plumping. HA’s water-binding ability produces a temporary plumping effect that can soften the appearance of fine lines. This is largely a surface phenomenon — the skin looks smoother because it’s better hydrated, not because HA is rebuilding collagen or reversing structural ageing. The effect is real but temporary. Stop using HA and the fine lines return within days as hydration drops back to baseline. That’s not a criticism — it’s a description of how humectants work.

Wound healing and barrier support. There’s a reasonable body of evidence, mostly from medical rather than cosmetic contexts, showing that HA supports wound healing and helps maintain the skin barrier. In skincare terms, that translates to a supporting role: HA helps an intact or recovering barrier hold onto moisture, which supports the conditions for repair. It’s not doing the repairing itself — that’s the job of ceramides, cholesterol, and fatty acids — but it creates a better environment for those processes to work in.

Anti-ageing beyond hydration. This is where claims start outrunning evidence. Some studies suggest that certain HA formulations can stimulate the skin’s own HA production or support collagen synthesis, but these findings are preliminary, often in vitro, and not yet convincingly replicated in the kind of well-controlled human trials that would make them reliable. If you’re looking for meaningful anti-ageing results, retinoids and vitamin C have far stronger evidence. HA is a hydration tool, not an anti-ageing treatment.

The dry-climate problem

Here’s something product labels almost never mention: in very dry or low-humidity environments, a humectant that draws water from the surrounding air has less water to draw. In these conditions, there’s a theoretical concern that HA could pull moisture from deeper skin layers instead, potentially leaving skin drier than it started. Whether this happens meaningfully in practice is debated — most formulations include occlusives that seal moisture in — but the logic is sound enough that it’s worth knowing about.

The practical fix is simple: apply HA to damp skin, not dry, and follow it with a moisturiser or occlusive layer to lock the water in. This isn’t a hack or a workaround — it’s just good practice with any humectant, and the reason why HA works best as part of a layered routine rather than on its own.

Sodium hyaluronate vs hyaluronic acid

You’ll see both on ingredient lists, sometimes on the same product. Sodium hyaluronate is the sodium salt form of HA — it’s smaller, more stable, and penetrates marginally better. In practical terms, the difference between the two in a finished product is minimal. Marketing that makes a fuss about which form a product uses is usually making a distinction without a meaningful difference. Both work. Neither is dramatically superior.

Using it well

Apply to damp skin — after cleansing, while your face is still slightly wet, or after a hydrating toner. Follow with a cream or oil to seal it in. This is especially important in drier climates or heated indoor environments. HA plays well with virtually everything: niacinamide, retinoids (yes, you can use them together — HA may actually help buffer irritation), vitamin C, AHAs, BHAs, peptides. It’s hard to build a bad combination around it.

Concentration matters less than you might expect. Most effective formulations sit between 0.1% and 2%. Higher concentrations can actually feel tacky and don’t necessarily deliver better hydration — the returns diminish fast, which is a recurring theme in skincare. Don’t choose a product based on who printed the highest percentage on the box.

Twice daily is fine. Morning and evening application is the most common approach, and consistent use gives better results than occasional heavy applications.

Who gets the most out of it

Dehydrated skin sees the clearest benefit — HA directly addresses the water deficit. Ageing skin benefits from the plumping and barrier support, even though HA isn’t an anti-ageing active in its own right. Sensitive or barrier-compromised skin generally tolerates HA well, and the hydration support aids recovery. If you’ve overdone it with actives and your barrier is struggling, HA is one of the gentler things you can reach for while it heals.

Where it does less: oily skin without dehydration may not notice much, since the skin’s already adequately hydrated. Deep wrinkles and significant photodamage need something stronger — HA softens their appearance temporarily but doesn’t address the underlying structural changes. And despite its reputation as universally tolerated, a small number of people do experience breakouts or irritation, possibly related to the low molecular weight fragment issue mentioned earlier. If you’ve tried HA and it doesn’t suit your skin, that’s a legitimate reaction, not an error on your part.

Common questions

Can I use hyaluronic acid with retinol? Yes, and many people find it helpful — the hydration can offset some of the dryness and irritation that retinoids cause, particularly during the adjustment period.

Does hyaluronic acid work on dry skin? It helps with dehydration (lack of water), but if your skin is dry (lack of oil), you’ll also need an emollient or occlusive. HA alone won’t solve dryness caused by insufficient lipid production — the distinction matters.

Is it safe during pregnancy? Topical HA is generally considered safe during pregnancy — it’s a substance your body already produces. As always, check with your midwife or GP if you’re uncertain, but this one doesn’t carry the cautions that retinoids do.

Should I use it morning, evening, or both? Both is fine. If you’re choosing one, morning makes sense if you’re layering it under sunscreen and makeup, since it provides a smooth hydrated base.

Does the percentage on the label matter? Less than you’d think. Effective concentrations start lower than most marketing implies, and going higher doesn’t proportionally improve results. A well-formulated 0.5% serum can outperform a poorly formulated 2% one.

The honest limitations

Hyaluronic acid does not penetrate deep enough to replace the HA your body is losing in the dermis with age. Topical application hydrates the outer layers of skin and improves surface appearance — that’s genuinely useful, but it’s not the same as reversing age-related HA depletion. Injectable HA (dermal fillers) does reach the dermis, but that’s a medical procedure, not a serum.

HA supplements exist, and some studies suggest oral HA may improve skin hydration, but the evidence is still early-stage and the mechanism — whether orally ingested HA reaches the skin in meaningful amounts — isn’t well established.

No topical humectant replaces a damaged barrier. If your skin isn’t holding onto water, the problem is usually the barrier itself — the lipid matrix that prevents water escaping — and HA doesn’t fix that. It adds water; the barrier keeps it there. Without both, you’re filling a leaky bucket.

Where that leaves you

Hyaluronic acid earns its place in a routine. It’s a reliable, well-tolerated humectant with solid evidence behind its main job: keeping the outer layers of skin hydrated. That translates to skin that looks plumper, feels more comfortable, and has a better environment for other products to work in.

What it isn’t is a miracle ingredient, an anti-ageing treatment, or a fix for structural skin concerns. It’s water management — good, honest, well-evidenced water management. Use it on damp skin, seal it in, don’t overthink the percentage, and pair it with actives that do the heavier lifting if you want results beyond hydration. That’s a modest verdict, but for an ingredient that genuinely delivers on its core promise, it’s enough.