Synopsis

Source of validity: Scientific
Rating (out of 5): 2

L-cystine, a dimer of the amino acid cysteine, is frequently included in oral supplements targeting hair loss or thinning. Its scientific basis stems from its integral role in keratin synthesis—the protein that makes up hair, nails, and the outer layer of skin. Several in vitro and small clinical studies have demonstrated that supplementation with L-cystine (often combined with other micronutrients such as B vitamins, zinc, and medicinal yeast) can have a positive effect on hair growth, especially in cases of telogen effluvium (diffuse hair shedding). For example, a randomized controlled trial published in the Journal of Cosmetic Dermatology (2018) reported that a supplement containing L-cystine, medicinal yeast, and pantothenic acid led to improved hair density and thickness compared to placebo over a six-month period. However, these studies tend to be limited by small sample sizes, short durations, and combined formulations, making it difficult to attribute benefits solely to L-cystine.

Despite these limitations, L-cystine’s inclusion in hair health products is supported by its biological role in hair structure and some clinical evidence—albeit moderate in quality. Its use is not based on historical or traditional medicine, but rather on its mechanistic plausibility and modern clinical research. Thus, while L-cystine may offer some benefit, expectations should be tempered by the current strength of the evidence.

More about L-cystine
More about Hair (loss or thinning)

Other ingredients used for Hair (loss or thinning)

Other health conditions supported by L-cystine

Products containing L-cystine

Thorne Amino Complex Berry
Thorne Amino Complex Lemon
Douglas Laboratories Free Form Amino
Klean Athlete Klean Isolate ™