Polyquaternium-51

Polyquaternium-51 1
Synthetic
Occlusive
Skin Conditioner
Clean
2-(N,N,N-Trimethylamino)ethyl 2-(2-methyl-2-propenoyloxy)ethyl phosphate inner salt, polymer with butyl 2-methyl-2-propenoate
CAS #
125275-25-4
Form
Powder
Functions
Film Forming
Creates a physical seal on skin to lock moisture in and keep irritants out. The "slugging" ingredient — barrier protection.
Skin Conditioning
A catch-all term for ingredients that improve how skin looks and feels — softer, smoother, more supple. The workhorse behind most moisturizers.
Locks in moisture
Traditional Use
A synthetic quaternary ammonium polymer developed in the late 20th century for use as a conditioning and film-forming agent in hair and skincare products. Designed to replace natural conditioning agents and improve product texture and performance.
EWG
1
CIR
Safe as used
Pregnancy
Yes
CIR Safety Findings
Not a Sensitizer
medical-approval
all-shades-safe
Regulatory Status
USA✔ permitted
EU⚠ restricted
JAPAN⚠ restricted
CANADA⚠ restricted
Skin Types
●all-skin-types
Best For
General
Pregnancy & Breastfeeding Safe
Yes
The paper trail
🔬Peer-reviewednone on file
⚖️Regulatory1 listing →
🛡️Safety-assessed2 reviews →
Counts, not a verdict. A listing isn't a study. The trail, you decide.
Verified Jun 2026
Data: CosIng · EWG · CIR