Polylysine: What Is It, Cosmetic Uses, Benefits & Side Effects

Curious about this ingredient? In this article we're explaining exactly what it is and why it's used within cosmetic formulations.
Updated on: July 1, 2025
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We verify all information on this page using publicly available nomenclature standards from The Personal Care Products Council (PCPC), the European Commission's CosIng database and documentation provided directly by ingredient manufacturers. Our analysis is based on technical data from these sources to ensure accuracy and reliability.

What Is Polylysine?

Polylysine is a chain of the natural amino acid lysine linked together into a smooth, flexible polymer. It is most often made by fermenting simple plant sugars with a friendly soil bacterium called Streptomyces albulus. After the fermentation broth is filtered and cleaned the result is a water-soluble powder that mixes easily into cosmetic bases.

The ingredient first drew attention in the food world where it was valued for keeping products fresh. Its safety record there encouraged formulators to try it on skin and hair in the early 2000s. They found that the positive charge of the lysine units helps the polymer cling lightly to hair fibers giving a soft, conditioned feel without heavy build-up. Because it is biodegradable and comes from renewable feedstocks it also fits the growing demand for greener beauty options.

Today polylysine appears in rinse-off and leave-on hair products such as shampoos, creamy masks, sprays and gentle styling creams. You may also spot it in multi-tasking skin gels that promise a silky after-feel though its main role is still in hair care.

Polylysine’s Cosmetic Benefits/Uses

Polylysine serves one key purpose in cosmetics: hair conditioning. The polymer forms an invisible film that smooths the cuticle, reduces static and helps hair strands slide past each other. This makes combing easier, boosts softness and adds a light touch of shine without weighing hair down. Its mild nature suits daily use and all hair textures including color-treated or chemically processed hair.

Who Can Use Polylysine

Polylysine is considered friendly for all skin and scalp types including dry, oily, sensitive and combination because it is water soluble and carries a mild positive charge that does not clog pores or leave a heavy residue. There are no known issues for acne-prone skin or conditions such as eczema although individuals with a proven allergy to lysine or other cationic polymers should stay cautious.

The ingredient is produced by fermenting plant sugars with a non-animal bacterium so it is suitable for vegans and vegetarians. No animal-derived processing aids are required and the finished polymer is readily biodegradable which appeals to eco-minded users.

Current safety data show no reproductive or developmental concerns, so products containing polylysine are generally regarded as safe for people who are pregnant or breastfeeding. That said this is not medical advice and anyone expecting or nursing should review new personal care products with a qualified healthcare professional first.

Polylysine does not absorb UV light or alter the skin’s response to the sun so it is not linked to photosensitivity. It is also odorless and unlikely to interfere with fragrance or colorants in a formula.

Potential Side Effects/Adverse Reactions

Reactions to topical polylysine can vary from person to person. The points below outline potential side effects that might occur although most users will not experience any problems when the ingredient is formulated and used correctly.

  • Mild skin or scalp irritation itching or tingling in sensitive individuals
  • Redness or inflammation usually short lived and linked to overuse of a concentrated product
  • Allergic contact dermatitis very rare but possible if someone is sensitized to lysine derivatives
  • Dryness or tight feel if the formula lacks complementary emollients
  • Build-up on hair may occur after heavy leave-in use without regular cleansing

If any persistent burning swelling or rash appears discontinue use and seek advice from a healthcare professional.

Comedogenic Rating

Rating: 0 (non-comedogenic). Polylysine is a water-soluble amino acid polymer that rinses away easily and contains no oily or waxy fractions that could clog pores. Its large molecular size keeps it on the surface rather than penetrating into follicular openings which further limits any risk of comedone formation. Because of this profile the ingredient is considered suitable for people prone to acne or breakouts. As a bonus it is usually used at low concentrations and often in rinse-off products which lowers residue even more.

Summary

Polylysine is mainly added to cosmetics as a light hair conditioner. Thanks to its positive charge it forms a thin film that smooths cuticles reduces static and lets strands glide so hair feels softer and looks shinier without weight. Although still a niche choice compared with silicones and quats its plant-fermented origin and biodegradable nature are fueling interest in clean beauty circles.

Current safety data show a low irritation rate no reproductive or developmental concerns and virtually zero comedogenicity making polylysine a low-risk ingredient for most users. That said everyone’s skin is unique so it is always smart to patch test any new product that contains polylysine before full use.

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