Hyaluronic Acid Serum: Complete Guide to Benefits and How to Use It

Hyaluronic acid (HA) is a naturally occurring polysaccharide found throughout your body — it's particularly concentrated in skin, joints, and eyes. One gram of HA can hold up to 6 liters of water, making it the most effective hydrating agent known in skincare.

Your skin's natural HA content decreases by about 50% between ages 20 and 50, contributing to the loss of plumpness and elasticity associated with aging.

Why Molecular Weight Matters

Not all hyaluronic acid in skincare works the same way — and this is where most products differ significantly.

• High molecular weight HA (>1,000 kDa): Cannot penetrate the skin. Sits on the surface, forming a hydrating film that prevents water loss. Provides immediate plumping effect.
• Medium molecular weight HA (100–1,000 kDa): Penetrates to the upper dermis. More lasting hydration effect.
• Low molecular weight HA (<100 kDa): Penetrates deeper into the dermis. Stimulates collagen production at deeper levels. Some research suggests potential irritation at very low weights.

The best products use multiple molecular weights for hydration at all depths. The Ordinary Hyaluronic Acid 2% + B5 (at $9) is one of the few affordable products that does this correctly.

The One Mistake That Makes HA Backfire

Best Hyaluronic Acid Products