Citrulline
INCI: CITRULLINE
Reviewed by Emma · Last updated 14 Aug 2026
An amino acid derivative used in skincare to support skin conditioning
Best For Skin Types
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Usage Frequency
Daily
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What Is Citrulline?
Citrulline is a non-essential amino acid that appears in skincare formulations as a skin conditioning agent. It is derived from or synthesised to mimic the amino acid found naturally in the body, where it plays a role in several physiological processes including nitric oxide production and protein synthesis.
In topical products, citrulline is primarily used to support skin hydration and overall skin condition. However, it remains a relatively niche ingredient, appearing in fewer than one in a thousand skincare products.
How It Works
Citrulline functions as a humectant and skin conditioning agent. As a humectant, it draws water into the outer layers of the skin, helping to maintain hydration. Additionally, as an amino acid, it may support the skin's natural protein structures, which are important for maintaining skin elasticity and barrier function.
The ingredient may also play a role in nitric oxide metabolism, though the significance of this in topical skincare remains unclear.
Clinical Evidence & References
Evidence for citrulline's efficacy in skincare is limited. The available research focuses primarily on systemic (oral or injectable) applications rather than topical use. Published studies on topical citrulline in skincare products are sparse, making it difficult to draw firm conclusions about its effectiveness at typical cosmetic concentrations.
What evidence exists suggests potential benefits for hydration and skin conditioning, but more robust clinical trials specifically examining topical formulations would be needed to establish clear efficacy profiles.
How to Use
Citrulline is typically found in serums, moisturisers and hydrating treatments. It works best as part of a layered skincare routine and pairs well with other hydrating and conditioning ingredients such as glycerin or hyaluronic acid. Apply to clean skin and follow with a moisturiser to seal in hydration.
Safety & Precautions
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