Thiamidol: melasma improved, freckles and sun spots did not

The 2026 Bangkok trial is the largest so far. 200 people with facial hyperpigmentation were randomised to 0.2% thiamidol cream or the same formulation without it for 12 weeks. It was double-blind, and blinded physicians scored mMASI from standardised photographs. 196 finished.

Reduction in mMASI (thiamidol vs vehicle):

• Week 4: 11.8% vs 5.4%
• Week 8: 27.9% vs 13.6%
• Week 12: 36.1% vs 16.1%

All three points at P < 0.001.

Note that the vehicle group also fell 16.1%. Applying a cream and staying out of the sun lowers pigment on its own. What thiamidol added is the 20 percentage points on top.

The 2021 24-week trial enrolled 48 people with moderate-to-severe melasma (23 thiamidol, 25 vehicle). MASI was significantly better than vehicle throughout the treatment period.

But that trial looked at one more thing. Participants came back 13 to 20 weeks after treatment ended. At that visit MASI was still lower than at baseline, but thiamidol and vehicle were similar to each other.

The gap that opened while people were applying it closed when they stopped. That is generally true of melasma treatment, but for this ingredient it has been confirmed in a trial.

Both trials were funded by the manufacturer that holds the patent. That is why we did not put this at 'strong evidence' under this site's rules.

The same 200-person 2026 trial also looked at pigment other than melasma.

For freckles and solar lentigines there was no statistically significant difference between the groups.

There is one more thing. mMASI is a physician's score, and the participants' own global assessments and the digital colour analysis showed no difference between groups. What the doctors could see was not picked up by the participants themselves or by the instrument.

Do not read this as 'it does not work'. Twelve weeks may be short for sun spots, and if the sample was recruited on melasma, people with freckles may have been a minority within it.

But 'good for dark spots' and what this trial found are not the same sentence. Melasma, freckles and solar lentigines arise differently, and what was confirmed here is melasma.

Hydroquinone is the long-standing first-line treatment for melasma, and in Korea it is a prescription medicine. So the practical question about this ingredient is whether it can stand in for it without a prescription.

A 2020 Brazilian trial ran that comparison. 50 women with facial melasma were randomised to 0.2% thiamidol twice a day or 4% hydroquinone cream at bedtime for 90 days. Evaluators were blinded, and both groups used an SPF60 tinted sunscreen.

Mean mMASI reduction:

• Thiamidol 43% (95% CI 35–50%)
• Hydroquinone 33% (23–42%)
• No difference between groups (P ≥ .09)

Thiamidol was ahead numerically, but not significantly. Failing to find a difference between two active treatments in 50 people does not establish that they are the same — it may be that there is no difference, or that there was no power to find one.

And here is what the trial reported on harms. Adverse effects in the thiamidol group were mostly mild, but allergic contact dermatitis appeared in two participants (8%). That is not unusual for the resorcinol family.

Keep in mind too that both groups used sunscreen. This trial does not show what happens without it.

In Korea, nine ingredients are listed in the functional cosmetics review rule (Annex 4) as accepted for a 'helps whiten skin' claim without submitting efficacy data of your own: broussonetia extract, arbutin, ethyl ascorbyl ether, licorice extract (oil-soluble), ascorbyl glucoside, magnesium ascorbyl phosphate, niacinamide, alpha-bisabolol and ascorbyl tetraisopalmitate.

Thiamidol is not on that list.

As on the cysteamine page, that does not mean it does not work — it means a product making the claim has to go through individual review.