Glycerin.

What it does

It draws water into the upper skin and helps it cling onto that moisture, and it supports healthy cell membranes, normal skin shedding, and barrier function.

Benefits

• Well-proven humectant that increases and holds skin hydration (measured glycerin levels correlate with skin hydration levels)
• Helps maintain the moisture barrier, supports normal shedding, and can leave a lasting 'reservoir' moisturizing effect
• Excellent for dry and barrier-compromised skin; glycerol is on the WHO List of Essential Medicines for atopic eczema and relieves xerosis and ichthyosis
• Extremely well tolerated, with essentially zero irritancy or comedogenicity

Side effects

• Feels sticky/tacky at high concentrations
• In very high concentrations (above roughly 40%) and in dry air it can paradoxically pull water out of skin; typical formulas use a much lower, well-tolerated level (often around 10%)

Who it's for

All skin types, especially dry, dehydrated, sensitive, or eczema-prone skin

Who should be careful

No meaningful contraindications at normal cosmetic concentrations