Hyaluronic acid (HA) is one of the most popular skincare ingredients in the world — and one of the most misunderstood. It's not an exfoliant. It's not an acid in the peeling sense. It's a humectant that holds water, and how well it works depends heavily on molecular weight and how you apply it. Here's how it actually functions.
- HA isn't one ingredient acting one way — molecular weight determines depth: high-MW sits on the surface for instant plumping, low-MW penetrates the epidermis for hydration from within.
- The most common mistake is applying it to completely dry skin in dry air, where it can pull water from deeper skin layers toward the surface faster than it evaporates gradually — always apply to damp skin and seal with a moisturizer.
- A 2024 randomized controlled trial found genuine efficacy differences between HA of different molecular weights for treating dry skin (xerosis), not just a theoretical distinction.
- Sodium hyaluronate, the salt form, has a smaller molecular size and is generally more stable in formulation and more effective at penetration than standard high-MW hyaluronic acid.
- HA works well in nearly every routine — it complements ceramides, niacinamide, retinol and vitamin C rather than competing with any of them.
What hyaluronic acid is
Hyaluronic acid is a polysaccharide naturally found in skin, connective tissue and the eyes. Its main function is to retain moisture — one gram of HA can hold up to six liters of water under lab conditions. In skincare, it's used as a humectant: it draws water from the environment and from deeper layers of skin toward the surface. It comes in several molecular weights, and each one behaves differently, penetrating to a different depth rather than all working identically.
Molecular weight matters
High molecular weight HA (roughly 1,000–1,800 kDa) sits on top of skin, forming a moisture film that plumps instantly but can't penetrate below the stratum corneum. Medium molecular weight penetrates slightly deeper. Low molecular weight HA (below roughly 50 kDa) reaches into the epidermis and supports hydration from within rather than just at the surface. Sodium hyaluronate — the salt form — has a smaller effective molecular size, tends to be more stable in formulation, and generally penetrates more effectively. A 2024 randomized controlled trial testing different HA molecular weights on dry skin (xerosis) in elderly patients found real, measurable efficacy differences between them, confirming this isn't just a formulation footnote. The best products combine multiple weights for both immediate plumping and longer-lasting hydration.
Who benefits most
Almost everyone, but especially: dry and dehydrated skin (a lack of water, distinct from a lack of oil), oily skin that is actually dehydrated underneath (more common than people expect — oily skin can still be low in water content), mature skin that has lost natural HA reserves with age, and skin recovering from harsh treatments or actives. Even oily skin generally benefits from proper hydration — addressing dehydration with HA often reduces the compensatory sebum overproduction that dehydrated oily skin tends to show.
Sodium hyaluronate vs. hyaluronic acid
Sodium hyaluronate is the sodium salt of hyaluronic acid. It has a smaller effective molecular size than standard high-MW HA, is more stable in formulations, and generally penetrates the skin more effectively as a result. Many well-formulated products use sodium hyaluronate instead of, or alongside, standard hyaluronic acid. When reading ingredient labels, both are genuinely good signs — the distinction matters more for formulators than for choosing between two otherwise-similar products.
Frequently asked questions
Can hyaluronic acid actually make my skin drier?
Yes, under specific conditions — this is a real, documented effect, not just internet folklore. In low humidity or on completely dry skin, HA can pull water from deeper skin layers toward the surface faster than dry air lets it evaporate gradually, leaving deeper skin relatively depleted. Always apply to damp skin and seal immediately with a moisturizer, especially in dry climates or heated indoor air.
Does the molecular weight of hyaluronic acid actually matter?
Yes — high molecular weight HA sits on the surface for instant plumping but can't penetrate below the stratum corneum, while low molecular weight HA reaches into the epidermis for hydration from within. A 2024 randomized trial on dry skin found genuine, measurable efficacy differences between HA of different molecular weights, not just a theoretical distinction.
Is sodium hyaluronate better than hyaluronic acid?
It's generally more stable in formulation and penetrates more effectively due to its smaller molecular size, but both are genuinely good ingredients to see on a label. The practical difference between products using one versus the other is usually smaller than the difference proper application technique makes.
How should I apply hyaluronic acid for the best results?
Apply it to slightly damp skin, right after cleansing or misting, then follow immediately with a moisturizer to seal the hydration in. Skipping the sealing step is the most common mistake — HA needs both available water to draw from and something to prevent that water from evaporating straight back out.
Is hyaluronic acid good for oily skin?
Yes, and it's often underused on oily skin specifically. Oily skin can still be dehydrated underneath — lacking water even while producing excess oil — and addressing that dehydration with HA often helps reduce the compensatory sebum overproduction that dehydrated oily skin tends to show.
Sources & further reading
- "Human skin penetration of hyaluronic acid of different molecular weights as probed by Raman spectroscopy" — ResearchGate / peer-reviewed
- "Efficacy Evaluation of a Topical Hyaluronic Acid Serum in Facial Photoaging" — Dermatology and Therapy, Springer Nature
- "Effectiveness of topical hyaluronic acid of different molecular weights in xerosis cutis treatment in elderly: a double-blind, randomized controlled trial" — PubMed, 2024
This article is written and edited by the Pure Moonlight team, grounded in the peer-reviewed and clinical sources listed above and cross-checked against our ingredient analyzer's database. It is not a substitute for advice from a dermatologist or doctor.