From Anirva, the maker of Hair Radiance. This article is general information, not medical advice. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.
Hair is made mostly of a protein called keratin, and keratin has an unusual recipe: up to 24% of its amino acids can be cystine. Cystine is made of two molecules of cysteine joined together. That is why L-cysteine appears in hair formulas — it is a building block that hair uses in large amounts. This article explains how cysteine fits into hair, what the human studies show, and how much is in Hair Radiance.
In this article
- Hair keratin is rich in cystine. Cystine may make up 24% of the amino acids in keratin. [1]
- Cystine is two cysteines joined together. The body can turn one into the other. [2]
- The links in cystine make keratin strong. Keratin owes its great stability to these links, called disulfide bonds. [1]
- Supplements with L-cystine have helped hair in two trials. In 30 women with hair shedding, one raised the share of growing hairs. In 40 women, another raised hair diameter. [3][4]
- Hair Radiance contains 150 mg of L-cysteine. Both trials tested blends, so the result cannot be credited to cysteine alone. [3][4][6]
Facts at a Glance
| Fact | Detail |
|---|---|
| Ingredient and form | L-cysteine, an amino acid [6] |
| Amount in Hair Radiance | 150 mg a day, in 3 capsules [6] |
| Percent of Daily Value | No Daily Value has been set [6] |
| Amounts used in studies | Not given in the trial reports. Both trials tested blends with L-cystine [3][4] |
| Kinds of evidence | Protein science; two human trials of blends with L-cystine; a review of a related form, N-acetylcysteine [1][2][3][4][5] |
| Human studies cited here | 2 trials: 30 women and 40 women. None of L-cysteine by itself |
| Adult upper limit | None has been set |
| Last reviewed | [DATE] |
What Is L-Cysteine?
L-cysteine is an amino acid, one of the building blocks of protein. It contains sulfur, which is what makes it special for hair.
Two molecules of cysteine can join to form cystine. Cystine can also be turned back into cysteine in the body. [2] So when you read about cysteine and cystine in hair research, they are two forms of the same building block.
Cystine is especially abundant in hair, horn and wool, and in skeletal and connective tissues. [2] Hair Radiance contains 150 mg of L-cysteine. [6]
How Does Cysteine Support Hair?
Hair keratin is rich in cystine
Each hair strand is built mostly from keratin. Hair keratin is a mix of proteins with high and low cystine content. [1] Across keratin as a whole, cystine may account for 24% of the amino acids. [1]
Cystine links hold keratin together
Cystine forms strong chemical links between protein chains, called disulfide bonds. The great stability of keratin comes from its many disulfide bonds. [1] Because keratin contains so much cystine, a growing hair needs a steady supply of it.
Cysteine helps the body make glutathione
N-acetylcysteine is a closely related form of cysteine. It supplies the body with sulfur groups and helps it make glutathione, an antioxidant that the body produces itself. [5]
Does L-Cysteine Help Hair Growth? What the Research Shows
The research on cysteine and hair is still growing. We found no trial of L-cysteine by itself for hair. Two human trials tested supplements that contained L-cystine, the joined form, together with other nutrients.
In 30 women with hair shedding, a supplement with L-cystine raised the share of growing hairs (2007)
This trial enrolled 30 healthy women with telogen effluvium, a common type of hair shedding. Half took a supplement for 6 months, and half took a placebo. The supplement combined L-cystine with medicinal yeast and B vitamins, including pantothenic acid. [3]
Researchers measured the share of hairs in the growth phase with a scalp imaging method. In the supplement group, this share returned toward normal within 6 months. The placebo group showed no significant change, and the difference between the groups was significant. [3] Independent raters also judged the photographs of the supplement group as better. [3]
The authors concluded that the supplement appears to push hairs into the growth phase. They also noted that its mode of action is not known. [3]
In 40 women, a supplement with L-cystine raised hair diameter (2015)
This trial enrolled 40 women with thinning hair for 180 days. The supplement combined a marine protein complex with L-cystine, L-methionine, vitamin C, silica from horsetail and other ingredients. [4]
In the supplement group, average hair diameter rose from 0.060 mm to 0.067 mm, and the number of full-size hairs increased. The placebo group showed no significant change. No adverse events were reported. [4] The company behind the supplement paid for the trial. [4]
| Study | What Was Used | Result |
|---|---|---|
| Lengg et al., 2007 [3] | L-cystine with medicinal yeast and B vitamins; 30 women; 6 months | Share of growing hairs returned toward normal; no change with placebo |
| Ablon and Dayan, 2015 [4] | A blend with L-cystine and L-methionine; 40 women; 180 days | Hair diameter up from 0.060 to 0.067 mm |
How Much L-Cysteine Is in Hair Radiance Compared With the Studies?
The two trial reports do not give their cystine amounts, so a direct comparison is not possible. No study has set an amount of cysteine for hair.
| Source | Amount a Day | Notes |
|---|---|---|
| Hair Radiance (3 capsules) | 150 mg of L-cysteine | From the label [6] |
| Trial in 30 women, 2007 | Not given in the report | L-cystine in a blend [3] |
| Trial in 40 women, 2015 | One tablet twice a day | L-cystine in a blend; amount not given [4] |
What the Research Does Not Show Yet
- We found no trial of L-cysteine by itself for hair.
- Both human trials tested blends, so they cannot show how much came from cystine. [3][4]
- The 2007 trial was small, with 30 women, and its report does not list the amount of each ingredient. [3]
- We found no study of cysteine combined with the other Hair Radiance ingredients.
Is L-Cysteine Safe? Safety and Interactions
Cysteine is a normal part of the protein in food. No upper limit has been set for it.
In the 180-day trial of a blend with L-cystine, no adverse events were reported. [4] Hair Radiance has 150 mg of L-cysteine a day. [6]
Hair Radiance is for adults. The product page advises people who are pregnant, nursing or taking prescription medicine to ask their doctor before use. [6]
L-Cysteine Is One Part of Hair Radiance
Hair Radiance is a daily three-capsule hair growth supplement with 22 ingredients: vitamins, minerals, plant extracts, an amino acid and three probiotic strains. They were chosen to support healthy hair growth, to help promote silky, smooth hair, and to strengthen and maintain healthy hair. L-cysteine is the amino acid among them.
Hair Radiance has not been clinically tested as a finished product. For this reason, we cannot say how its ingredients work together. Every ingredient and amount is listed on the label. [6]
L-cysteine is an amino acid that hair keratin uses in large amounts. [1][2] A few points can help you use it well: take Hair Radiance with food, as the directions say. [6] Give it time — the two trials ran for 6 months. [3][4] And if you are pregnant or nursing, or if you take prescription medicine, ask your doctor or pharmacist before you start. [6]
If You Are Deciding
Here is the case in brief. Hair is made mostly of keratin, and keratin is unusually rich in cystine, the joined form of cysteine. [1][2] The links in cystine are what make keratin stable. [1] In two human trials, supplements with L-cystine raised the share of growing hairs and the hair diameter. [3][4] Both were blends, so cysteine by itself has not been tested for hair. Hair Radiance supplies 150 mg of L-cysteine a day. [6] The directions are three capsules a day with food. [6]
About Hair Radiance
Each 3-capsule serving of Hair Radiance includes 150 mg of L-cysteine, alongside vitamins, minerals, plant extracts and three probiotic strains. Hair Radiance hasn't been clinically tested as a finished formula. The label lists every ingredient and its amount.
See the Hair Radiance product page →The Short Version
Hair keratin is unusually rich in cystine, which is two cysteines joined together. The links in cystine give keratin its strength. In two trials, supplements with L-cystine helped hair, but both were blends. Hair Radiance provides 150 mg of L-cysteine a day.
Frequently Asked Questions
References
- Koshland DE. Protein: Keratin. Encyclopaedia Britannica. Last updated September 18, 2026.
- Gregersen E, Britannica Editors. Cystine. Encyclopaedia Britannica.
- Lengg N, Heidecker B, Seifert B, Trüeb RM. Dietary supplement increases anagen hair rate in women with telogen effluvium: results of a double-blind, placebo-controlled trial. Therapy. 2007;4(1):59-65. doi:10.2217/14750708.4.1.59.
- Ablon G, Dayan S. A randomized, double-blind, placebo-controlled, multi-center, extension trial evaluating the efficacy of a new oral supplement in women with self-perceived thinning hair. J Clin Aesthet Dermatol. 2015;8(12):15-21.
- Kelly GS. Clinical applications of N-acetylcysteine. Altern Med Rev. 1998;3(2):114-127. PMID: 9577247.
- Anirva. Hair Radiance Supplement Facts label and product page.
