Ingredient review

PEI-45

INCI: PEI-45

PEI-45 is a synthetic film-former that helps products like mascara and hair gels stay put, but it's not a hydrating or active ingredient.

beautyskincarefilm-former

In plain English

PEI-45 is a man-made polymer that creates a thin, flexible film on skin, hair, or nails. Think of it like a clear, invisible glue that helps makeup or styling products last longer without flaking. It's not a moisturizer or active ingredient—it's more of a helper that improves texture and wear.

Review score

Safety, usefulness, and evidence

3.5Good
3.5/ 5

Potentially useful with some tradeoffs

The evidence base is useful, but some claims depend heavily on the formula.

Risk flags are low for most users, though the finished product can still irritate.

Source
synthetic
Evidence
moderate
Irritation
low
Clogging risk
low
How reviews are scored

Quick decision guide

Useful, but context matters

PEI-45 is generally a lower-concern ingredient when the full formula suits your skin.

Plain-English read

Treat this as a practical screening step before you compare products that contain this ingredient.

  1. Step 1Start with the score, then check the irritation and clogging risk before judging PEI-45.
  2. Step 2Use the "Best for" and "Use caution if" sections to match the ingredient to your skin, not just to a marketing claim.
  3. Step 3If a product stings, breaks you out, or worsens irritation, judge the finished formula and stop using it even if the ingredient scores well.

Score terms in plain English

Irritation risk

low

Less likely to sting, burn, or bother most users, though sensitive skin can still react.

Clogging risk

low

Less likely to feel heavy or contribute to clogged pores for most skin types.

Evidence level

moderate

There is useful support, but formula details and claim strength still matter.

What it is

PEI-45 is a polyethyleneimine polymer, a synthetic compound that forms a film when applied. It's often used in color cosmetics and hair products to improve adhesion and water resistance.

How it works

When applied, PEI-45 molecules link together to form a continuous, flexible film on the surface. This film helps other ingredients stick better, reduces smudging, and can make formulas feel smoother or more spreadable.

Pros

Long-lasting wear

PEI-45 helps mascara, eyeliner, and hair gels resist smudging and flaking throughout the day.

Smooth application

It improves the spreadability of formulas, making them easier to apply evenly without clumping.

Cons and cautions

Not hydrating

PEI-45 is a film-former, not a moisturizer. It won't add hydration or soothe skin.

Potential heaviness

In high concentrations, it can feel stiff or heavy on lashes or hair, especially if layered.

Best for

  • Anyone wanting long-wearing mascara or eyeliner
  • People who prefer smudge-proof hair styling products

Use caution if

  • Those with very sensitive or reactive skin (patch test recommended)
  • People seeking purely natural or plant-based formulas

Usage tips

Use in small amounts to avoid a stiff or heavy feel.
Pair with hydrating ingredients in the same routine to balance any drying effect from the film.
Shake or stir products containing PEI-45 well before use to ensure even distribution.

Safety summary

PEI-45 is considered safe for cosmetic use at typical concentrations. It has low irritation and comedogenic potential, but those with very sensitive skin should patch test first.

Research notes

Research on PEI-45 is limited compared to more common film-formers. Available studies focus on its film-forming ability and safety in topical applications, with no major red flags.

Common label clues

Typical concentration
Typically used at low concentrations, often under 5%
Regulatory status
Approved for use in cosmetics in the US and EU at typical concentrations. No specific restrictions, but always follow manufacturer guidelines.
Common uses
Mascara, Eyeliners, Hair styling products, Nail treatments
Environmental note
As a synthetic polymer, PEI-45 is not biodegradable. Its environmental impact depends on formulation and disposal.

Good to know

  • PEI-45 is often listed near the end of ingredient lists because it's used in low concentrations.
  • It's not the same as polyethylene glycol (PEG) or other common polymers—each has different properties.

Common questions

What is PEI-45 in beauty products?

PEI-45 is a man-made polymer that creates a thin, flexible film on skin, hair, or nails. Think of it like a clear, invisible glue that helps makeup or styling products last longer without flaking. It's not a moisturizer or active ingredient—it's more of a helper that improves texture and wear.

What does PEI-45 do in a beauty product?

When applied, PEI-45 molecules link together to form a continuous, flexible film on the surface. This film helps other ingredients stick better, reduces smudging, and can make formulas feel smoother or more spreadable.

Is PEI-45 safe for most people?

PEI-45 is considered safe for cosmetic use at typical concentrations. It has low irritation and comedogenic potential, but those with very sensitive skin should patch test first.

Who should be careful with PEI-45?

Those with very sensitive or reactive skin (patch test recommended) People seeking purely natural or plant-based formulas

Research sources

Cosmetic Ingredient Review (CIR) - Safety Assessment of PolyethyleneimineOpenPubChem - PolyethyleneimineOpenEuropean Chemicals Agency (ECHA) - PolyethyleneimineOpen

Ingredient reviews are educational and are not medical advice. Patch test new products and ask a licensed clinician about persistent irritation, allergies, pregnancy-specific questions, or diagnosed skin conditions.