Encyclopedia · chemical treatment
Glyoxylic acid
Marketed as a safe "formaldehyde-free" straightening agent, but it has been linked to acute kidney failure and the EU has begun a safety review.
What it is
A small aldehyde acid used in "Brazilian", "progressive" or "keratin" straightening, at up to about 16% (the concentration a 2026 EU safety mandate asks about). It is a cosmetic ingredient with no restriction yet, but the European Commission has asked its scientific committee to assess it.
How it works
It does not break disulfide bonds. It appears to alter hair protein, giving straightening that fades. Velasco 2022 Skin absorption followed by metabolism to oxalate is the presumed route to kidney injury, as shown in mice. Robert 2024
What the studies show
- A patient had three episodes of acute kidney injury after Brazilian straightening treatments. In mice, topical glyoxylic acid and the product caused calcium oxalate crystal kidney damage. Robert 2024
- On hair swatches, glyoxylic acid caused more protein loss (3.5 ug/g) than alkaline or thio products (about 2.5), with similar break force. Velasco 2022
- French authorities raised an alert in October 2024 about four kidney failure cases, and the European mandate was published on 4 May 2026.
How it's used in studies
In salons it is applied, then blow-dried and flat-ironed at high temperature.
Side effects and safety
Serious. Acute kidney injury has been reported, and heat may release formaldehyde. Avoid it if you have kidney disease, are pregnant (precaution; no data) or react to formaldehyde. Check the full product ingredient list, because glyoxylic acid has also turned up undeclared in some products.
Bottom line
Not recommended while the EU review is pending. "Formaldehyde-free" does not mean safe. Evidence on both efficacy and safety is thin, and the real-world kidney risk is not yet known. See relaxers, perms and smoothing.
Studies 2
Robert T et al. Hair-straightening cosmetics containing glyoxylic acid induce crystalline nephropathy. Kidney Int. 2024
Tested: glyoxylic acid hair straightener exposure
Result: Glyoxylic acid straightener use was linked to crystal-forming kidney damage in a patient, and the damage was reproduced in mice.
Read the study ↗Velasco MVR et al. Impact of acid (Progressive Brush) and alkaline straightening on the hair fiber. Int J Trichology. 2022
Tested: sodium hydroxide, guanidine hydroxide, thio and glyoxylic acid straighteners
Result: Alkaline relaxers and thio raised protein loss to about 2.5 ug/g versus 1.12 untreated; glyoxylic acid reached 3.5; break force was similar across products.
Read the study ↗Related
- Formaldehyde and formaldehyde releasers (methylene glycol)The cross-linker behind classic keratin smoothing; it works but releases a human carcinogen when heated, and even "formaldehyde-free" versions can exceed workplace limits.
- Sodium hydroxide and guanidine relaxers (lye and no-lye)Chemical relaxers permanently straighten tightly curled hair; they work, but are linked to scalp burns, breakage and cancer associations still being investigated.
- Relaxers, perms and smoothing treatmentsChemical straighteners and perms all damage hair, and some carry scalp, respiratory, kidney or cancer-association concerns worth knowing before you book.