Topical hyaluronic acid provides modest, immediate skin hydration by binding water at the surface. Injected hyaluronic acid (as a filler) provides real dermal volume. These are meaningfully different products with different evidence bases.
Topical HA. The mechanism is straightforward — hyaluronic acid is a highly hygroscopic molecule that can bind up to 1,000 times its weight in water. Applied topically, it holds water at the stratum corneum, producing an immediate plumping effect and modest short-term hydration improvement (Papakonstantinou, 2012).
What topical HA does not do: penetrate meaningfully into the dermis, stimulate endogenous HA production, or produce lasting structural change. Molecular weight varies substantially across products — smaller fragments claim deeper penetration but the evidence for skin structural effects is limited.
Practical: pleasant to use, safe, adds surface hydration especially in low-humidity environments. Layer under moisturizer, apply to damp skin. Not a replacement for retinoids or actives that produce structural change.
Injected HA fillers. A completely different product. Cross-linked HA gel injected into the dermis produces immediate structural volume that lasts 6-18 months depending on product and location. Well-established evidence for facial volume restoration, tear troughs, lip enhancement.
The confusion between topical and injected HA is one of the most-cited examples of marketing conflating two different products.