Encyclopedia · chemical treatment
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.
What it is
Lye relaxers use sodium hydroxide. "No-lye" relaxers use guanidine hydroxide, formed from calcium hydroxide and guanidine carbonate. Both are cosmetic products, with the stronger formulations sold professionally, and both have a pH over 12.
How it works
A strong alkali swells the fibre and rearranges its internal cross-links, which permanently relaxes the curl. A Brazilian review of 33 studies reported eczema, burns, inflammation, hair loss and shaft damage. Hatsbach de Paula 2022
What the studies show
- A review of 33 studies linked relaxers with scalp eczema, burns, hair loss and shaft damage; long-term effects are unknown. Hatsbach de Paula 2022
- On hair swatches, sodium and guanidine hydroxide each caused about 2.5 ug/g protein loss versus 1.12 in virgin hair. Velasco 2022
- In the US Sister Study (33,947 women, 378 uterine cancers), straightener use in the last 12 months had a hazard ratio of 1.80 and use more than four times a year 2.55 (1.46 to 4.45). Hair dyes were not associated. Chang 2022
- In the Black Women's Health Study (44,798 women, 347 cases), heavy use had a hazard ratio of 1.18 (0.81 to 1.71) overall, and about 1.6 in postmenopausal women with moderate or heavy use. Bertrand 2023
- A 2026 Sister Study analysis of 46,287 women reported thyroid cancer (HR 1.71) and pancreatic cancer (HR 2.66), with little dose response; the authors say confirmation is needed. Bailey 2026
- A 2026 scoping review of 21 studies found relaxers contain endocrine-disrupting chemicals (phthalates, phenols, parabens) and fragrance ingredients, while keratin treatments contain more formaldehyde. Weaver 2026
How it's used in studies
Relaxers are applied by professionals or at home with a base to protect the scalp, left on for minutes and then neutralised. There is no standard protocol.
Side effects and safety
Serious: chemical burns and hair loss, and swallowing is a medical emergency. The cancer associations are observational and not proof of cause, and confounding is possible. We found no verified pregnancy data. Retouch intervals and strength matter for damage.
Bottom line
Relaxers are effective cosmetically, but the evidence does not support any claim of safe use. Consider reducing frequency, keeping product off your scalp and asking about alternatives such as a silk press or heat-free stretching. We are moderately sure about the harms and unsure about cancer causality. See relaxers, perms and smoothing.
Studies 6
Hatsbach de Paula JN et al. Effects of chemical straighteners on the hair shaft and scalp. An Bras Dermatol. 2022
Result: Relaxers were linked to scalp eczema, burns, inflammation, hair loss and shaft damage; long-term effects remain unknown.
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 ↗Chang CJ et al. Use of straighteners and other hair products and incident uterine cancer. J Natl Cancer Inst. 2022
Tested: hair dye, straighteners, perms
Result: Hair dye was not linked to uterine cancer; straightener use was (HR 1.80, 2.55 with frequent use). Observational; does not isolate a chemical.
Read the study ↗Bertrand KA et al. Hair relaxer use and risk of uterine cancer in the Black Women's Health Study. Environ Res. 2023
Tested: hair relaxer use
Result: No overall increase for heavy use (HR 1.18, CI 0.81-1.71), but higher risk in postmenopausal women (HR about 1.6-1.7).
Read the study ↗Bailey JT et al. Use of hair straighteners and chemical relaxers and incidence of non-reproductive cancers. J Natl Cancer Inst. 2026
Result: Higher thyroid (HR 1.71) and pancreatic (HR 2.66) cancer with little dose response; the authors called for confirmation.
Read the study ↗Weaver E et al. Chemicals of concern in chemical hair straightening products: a scoping review. Curr Environ Health Rep. 2026
Result: Keratin treatments more often contained formaldehyde or releasers; relaxers more often contained endocrine-disrupting chemicals and fragrance ingredients.
Read the study ↗Related
- Ammonium thioglycolateThe classic reducing agent in perms and thio relaxers; it breaks hair's disulfide bonds so the shape can change, with some loss each time.
- Glyoxylic acidMarketed as a safe "formaldehyde-free" straightening agent, but it has been linked to acute kidney failure and the EU has begun a safety review.
- 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.
- 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.