Polyquaternium-51
No meaningful concern for most people; irritation is possible but rare.
Also written as: MPC Polymer · Lipidure · Poly(2-methacryloyloxyethyl phosphorylcholine)
What it is
Polyquaternium-51 is a polymer built from a unit that mimics phosphorylcholine, the head group found on the outside of human cell membranes. That structure binds water extremely well and is very compatible with biological surfaces. In cosmetics it forms a light, breathable moisture-retaining film, and it is common in Japanese and Korean sheet masks and essences.
What it does well
Holds water at the skin surface for extended periods
Its phosphorylcholine structure gives strong water binding and it is widely used for this purpose, but independent published clinical trials on skin hydration are scarce.
Limited evidence
Worth knowing
Very little independent published data
Most information comes from the raw material supplier rather than peer-reviewed studies. There is no specific safety signal, and it has a long record of use in Japanese cosmetics.
Matters most toAnyone who prefers ingredients with independent published evidence.
Where the EU stands
No specific restriction
Not listed in any restriction or prohibition annex of the EU Cosmetics Regulation; permitted at the formulator's discretion under the general safety requirement.
