Cysteine — Allergy Risk & Safety Profile
INCI: CYSTEINE
Key Facts
- CAS Number
- 52-90-4
- Category
- amino acid
- Risk Level
- low
- Common Allergen
- No
- Found In
- hair treatment
Is Cysteine safe in cosmetics?
Cysteine is rated low risk in cosmetic products. It is not classified as a common allergen and is generally well-tolerated by most skin types. Commonly found in hair treatment.
What is Cysteine?
Cysteine is a amino_acid commonly used in cosmetic and personal care products. It is generally considered safe at recommended concentrations.
Allergy & Safety Information
Cysteine is considered safe for cosmetic use.
This ingredient is considered low risk from an allergenicity perspective. Reactions are uncommon at typical cosmetic use concentrations. However, no ingredient is universally safe — if you notice redness, itching, or irritation after using a product containing this ingredient, discontinue use and monitor whether symptoms resolve.
How to Identify Cysteine in Products
Cosmetic ingredients are listed by their INCI (International Nomenclature of Cosmetic Ingredients) name in descending order of concentration. To check whether a product contains Cysteine, scan the ingredient list for its INCI name exactly as it appears above. Ingredients present at 1% or less may appear in any order after the 1% threshold.
Product types to prioritise checking: Hair treatment. These product categories are the most common sources of Cysteine exposure based on formulation data.
Not sure if your products contain Cysteine? Paste the ingredient list into the free ingredient checker to scan for Cysteine and other allergens automatically.
Frequently Asked Questions
Is Cysteine safe?
Yes, well-tolerated by most skin types.
What does Cysteine do?
Functions as a amino_acid in formulations.
Related Guides
Is Cysteine causing YOUR skin reactions?
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