Ingredient Beauty BenefitsAntioxidant · Cysteamine HCl
Cysteamine: pool seven randomised trials and it beat placebo and tied with hydroquinone
A 2024 meta-analysis pooled seven randomised controlled trials. Cysteamine 5% significantly reduced melasma scores versus placebo (SMD −0.84, 95% CI −1.19 to −0.49, I² = 0%) and showed no significant difference against hydroquinone 4% (SMD 0.16, P = .42). The individual trials, though, ran 20 to 40 people.
Last checked Sep 18, 2026
- In pregnancy
- Not established
- Irritation
- High
- Daytime use
- ✗ Night only
Not a measurement — reported irritation grouped into five steps. It varies between people.
Cysteamine is an aminothiol. The body produces it naturally when breaking down cysteine, and it was originally used as a treatment for a kidney condition (cystinosis).
One thing makes it stand out on this site: for a cosmetic ingredient, the shape of its evidence is properly formed.
Two problems recur throughout this layer — either the comparator is another product, so 'neither worked' cannot be ruled out, or there is a single trial, so chance cannot be ruled out. Cysteamine has placebo-controlled trials, a meta-analysis pooling them, and head-to-head comparison against the established first-line treatment.
We still did not raise it to 'strong evidence'. The reasoning is at the end of the first item below; read it, and if you think we were too strict, read the page that way.
For melasma, read alongside tranexamic acid, arbutin and niacinamide to see the options.
Contents
What is established
The same ingredient at a different strength, in a different formula, is a different product. These are the conditions the studies used.
Pooling seven randomised trials, it beat placebo
Start with the 2024 meta-analysis. It searched Europe PMC, Medline, Scopus and the Cochrane Library up to 22 June 2024 for randomised controlled trials comparing cysteamine with placebo or hydroquinone. Seven qualified.
Against placebo:
- Melasma severity (MASI): standardised mean difference −0.84
- 95% CI −1.19 to −0.49
- P < .00001
- I² = 0%
The I² = 0% is the most striking number here. I² expresses how much the pooled trials disagree with each other, and 0% means they do not disagree at all. That is an unusual figure for an ingredient in this layer.
The most important single trial is a 2018 double-blind, placebo-controlled study in Iran. Forty people applied placebo (20) or cysteamine cream (20) every night for four months. Dermacatch and Mexameter colorimetry, MASI scores and Investigator Global Assessments were all used.
The difference between pigmented and normal skin (by Dermacatch) moved like this:
- Cysteamine: 72.3 → (2 months) 38.1 → (4 months) 23.8
- Placebo: 52.9 → (2 months) 64.9 → (4 months) 50.0
The groups differed significantly at both time points (P = .01, P = .02), and MASI at four months was 8.03 versus 12.2 (P = .04).
The weakness, stated as it stands. Baseline pigmentation already differed: 72.3 in the cysteamine group against 52.9 in the placebo group. The cysteamine group started more pigmented. Randomisation does not prevent imbalances like this at twenty per arm. Since a darker starting point leaves more room to fall, the imbalance can work in cysteamine's favour.
So why is this not 'strong evidence'?
By this site's own rule it could be: seven distinct randomised trials, an instrument endpoint, and not all industry-funded. It passes our lint.
We did not raise it for one reason. The seven pooled trials run 20 to 40 people each. On this site 'strong' is where sunscreen and prescription retinoids sit, and those are trials of hundreds to thousands. Give this the same badge and readers will give it the same weight.
Many small trials is not the same thing as several large ones. A meta-analysis adds the participants together; it does not wash out a bias that all the small trials share.
- Randomised controlled trial in people · 40 participants · 16 weeks · Compared with vehicle (same formula minus the active) · Funding not declared · Endpoint instrument measurement PMID 28678558
- Randomised controlled trial in people (7) · sample size not reported · Compared with vehicle (same formula minus the active) · Funding not declared · Endpoint instrument measurement PMID 39673630
It tied with hydroquinone — and with kojic acid
Hydroquinone is the long-standing first-line choice for melasma. In Korea it is a prescription-only medicine. Half the interest in cysteamine sits right here: can it be an alternative you can use without a prescription?
The 2024 meta-analysis made this comparison too.
- Cysteamine 5% versus hydroquinone 4%: standardised mean difference 0.16
- 95% CI −0.22 to 0.53, P = .42
- I² = 55%
No significant difference. But unlike the placebo comparison, I² is 55% — the trials disagreed with each other considerably, and that calls for correspondingly more caution.
Take one trial in detail: a 2021 randomised, double-blinded study in Australia. Twenty participants used cysteamine cream or hydroquinone cream for 16 weeks.
Before the result, something has to be said first: only 14 of the 20 completed, and those 14 split 5 cysteamine to 9 hydroquinone.
mMASI fell 21.3% with cysteamine and 32% with hydroquinone, and the difference between groups was not significant (P = .3).
That 'no difference' must not be read as 'the same'. Failing to find a difference between five people and nine may mean there is none, or may mean there was no power to find one. The authors themselves wrote that larger studies are needed. On the raw numbers, hydroquinone fell further.
In the same trial, hydroquinone was generally better tolerated. Side effects were more common with cysteamine, though mild and reversible.
A 2025 trial in India compared it with kojic acid. Seventy-two women were randomised 1:1 to 5% cysteamine or 2% kojic acid nightly for 16 weeks. mMASI fell 12.25% versus 10% and melanin content 5.13% versus 4.81% — cysteamine ahead on the numbers, but not significantly.
That trial was open-label: participants and investigators both knew which arm they were in. And how visible a 10 to 12% fall in mMASI over 16 weeks actually is remains a separate question.
- Randomised controlled trial in people · 20 participants · 16 weeks · Compared with another active ingredient · Funding not declared · Endpoint scores from blinded assessors PMID 32981068
- Randomised controlled trial in people · 72 participants · 16 weeks · Compared with another active ingredient · Funding not declared · Endpoint scores from blinded assessors PMID 40296942
Cysteamine is not among Korea's nine notified brightening ingredients
In Korea, the ingredients accepted as 'functional cosmetic — helps brighten the skin' without efficacy data are listed in Annex 4 of the Regulation on the Review of Functional Cosmetics. There are nine:
Broussonetia extract, arbutin, ethyl ascorbyl ether, oil-soluble licorice extract, ascorbyl glucoside, magnesium ascorbyl phosphate, niacinamide, alpha-bisabolol and ascorbyl tetraisopalmitate.
Cysteamine is not on that list.
The same point as on the retinaldehyde page: it does not mean the ingredient does not work, it means a functional claim requires individual review.
One thing more is specific to this ingredient. Every trial above used 5%. Whether a cysteamine product sold in Korea is at that concentration, and under which regulatory category it is governed, is something to check product by product. What we verified from source documents stops at 'it is not on the notified list'.
How to use it
- Every trial above used 5% cream, once daily at bedtime.
- The trials ran 16 weeks (four months). A difference appeared by two months but widened by four.
- It smells strong. It is a sulphur-containing molecule with a characteristic odour, which is why many products are formulated for a short contact time and then rinsing off. Follow the product's directions.
- Sunscreen has to come with it. Melasma darkens again with sun. Skip that one thing and little of the rest survives.
- Niacinamide is a notified brightening ingredient and far less irritating. Using both together is a natural arrangement.
Cautions
- Erythema, irritation, burning, itching and dryness were more common than with placebo. The meta-analysis lists these directly.
- Against hydroquinone, adverse event rates were similar.
- The characteristic smell is a common reason people stop.
- We found no established data on use during pregnancy.
- Melasma recurs. None of the trials above looked at how long the improvement holds after stopping.
With a new ingredient, apply a small amount somewhere inconspicuous, such as the inner arm, for two days first. Stop if it reddens or stings.
Related skin concerns
What this article relies on
- PubMed — 미국 국립의학도서관 문헌 데이터베이스National Library of Medicine · Checked on 2026-09-16
- 기능성화장품 심사에 관한 규정 [별표 4] 자료제출이 생략되는 기능성화장품의 종류 — 식품의약품안전처고시 제2025-88호식품의약품안전처 · 시행 2025-12-16 · Checked on 2026-09-16