Type "keratin hair" and you fall into two worlds that talk without understanding each other: salons that offer keratin smoothing keratin smoothing, and supplements that offer keratin in capsules. The word is the same. The molecule, at its base, is the same. And yet these two things don't work on the same hair, not in the same place, not in the same timeframe, and one of the two has a formaldehyde problem that the other doesn't.
This article clears up the confusion once and for all. What a smoothing treatment does, with the formaldehyde dosages measured product by product in two studies, including in products labeled "formaldehyde-free." What oral keratin does, with its two randomized trials. What each can and cannot promise. And how to combine them intelligently, because the answer isn't "one or the other."
The smoothing treatment. Application of hydrolyzed keratin and a fixative to the existing hair fiber, sealed with a flat iron at over 200 degrees. Smooth and shiny effect, immediate, two to four months. The effective fixative is formaldehyde, a known carcinogen: dosed at 0.96 to 1.4% in six out of seven brands in South Africa, five of which labeled "formaldehyde-free," and up to 11.5% in one American professional product. In salon air, up to 3.47 ppm during blow-drying. In 43 patients, scalp reaction with positive pull test in 86%.
Oral keratin. A keratin solubilized is digested and nourishes the follicle that produces the next hair. Internal effect, three months, on new fiber: shedding, growth, strength, shine, nails, in two randomized double-blind trials. The verdict. Smoothing treatment dresses the existing hair, oral keratin builds the hair to come. They combine, if the smoothing treatment is compliant.
- Same word, two hairs: the one that has emerged and the one that is growing
- Smoothing treatment: how it holds, and what makes it last
- Formaldehyde, product by product
- What smoothing treatment does to the scalp and fiber
- Oral keratin: the follicle, and two trials
- Choose, or combine
- Frequently asked questions
Same word, two hairs: the one that has emerged and the one that is growing
Hair has two lives. The first takes place under the skin, in the follicle, where some of the fastest-growing cells in the body assemble keratin from amino acids delivered by the blood. The second begins as the hair exits the scalp: the fiber is then a deadstructure, without living cells, without blood vessels, which does not repair itself and does not receive nourishment. What happens to it after that—heat, coloring, friction—accumulates until it is cut.
| Keratin Straightening | Keratin in Capsules | |
|---|---|---|
| Acts on | The fiber already emerged, dead | The follicle, living, which produces the next fiber |
| Where | On the surface, on the cuticle | Inside, within the structure of the new fiber |
| How | Deposit of hydrolyzed keratin and a fixative agent, sealed with heat | Digestion, absorption of sulfur-containing amino acids, delivery to the follicle |
| Timeline | Immediate | Three months, the time it takes for new hair to emerge |
| Duration | Two to four months, diminishes with shampooing and new growth | As long as the follicle is nourished; the fiber built remains |
| Changes hair shape | Yes, that's its purpose | No, the shape is determined by the follicle |
| Changes hair strength | No, and heat reduces it | Yes, on the new fiber, measured in testing |
Everything stems from this distinction: straightening works on presenthair, oral keratin on hair to come. Expecting straightening to strengthen hair, or a capsule to straighten it, is asking each to do what only the other can accomplish.
Straightening: how it holds, and what makes it last
A keratin straightening treatment, also called Brazilian straightening, proceeds in three steps. A clarifying shampoo opens the scales of the cuticle. A preparation is applied strand by strand: it contains hydrolyzed keratin, conditioning agents, and a fixative. Then the hair is dried with a hair dryer and run through a flat iron at 200 degrees or higher, which triggers the fixative's reaction: it creates bonds between the keratin deposited and that of the fiber, and sets the hair in its smooth position.
The keratin in the product alone would not straighten anything and would not hold: it would wash out at the first shampoo. What makes a straightening treatment last two to four months is the fixative, and the most effective fixative ever found is formaldehyde, or molecules that release it when heated, with methylene glycol leading the way. The word "keratin" on the salon door is the reassuring part of the formula; formaldehyde is the part that works.
Compliant straightening treatments exist, formulated with other fixatives such as glyoxylic acid. They generally last shorter and straighten less drastically: this is the price of compliance, and it is reasonable.
Formaldehyde, product by product
<<<14>>> Formaldehyde formaldéhyde is classified as a definite carcinogen for humans by the International Agency for Research on Cancer. It is also irritating to the eyes, nose, throat, and skin, and allergenic. The maximum concentration deemed safe in a cosmetic by the American expert panel is 0.2%. In Europe, it has been banned in cosmetics since 2019, and substances that release it must be labeled above a certain threshold.
In the bottles: seven brands, one country
A dermatology team from Cape Town measured by chromatography, three times each, all Brazilian straightening brands sold in South Africa in 2012.
| Result | Figure |
|---|---|
| Brands analyzed | 7 |
| Brands containing formaldehyde | 6 out of 7 |
| Concentrations measured | 0.96 to 1.4%, or five to seven times the limit deemed safe |
| Brands labeled "formaldehyde-free" among the six | 5 |
The authors conclude that the concentrations in these products may exceed recommended levels and constitute a health hazard for hairdressers and clients, and call for industry oversight. The figure that matters is the last one: five products out of six that contained it claimed not to contain it. The label, in this market, was not informative.
In the salon air: four professional products
A second team recreated a salon day: four American professional products applied to human hair wigs, 88 air samples at different distances, measurement of formaldehyde in the products themselves.
| Product | Formaldehyde in the bottle |
|---|---|
| Product A | 11.5% |
| Product B | 8.3% |
| Product C | 3% |
| Product D | 0%, but precursors that degrade into formaldehyde when heated |
| In the air | Concentration |
|---|---|
| During blow-drying | 0.08 to 3.47 ppm |
| During flat ironing | 0.08 to 1.05 ppm |
| Daily exposure of the hairstylist, one straightening treatment per day | 0.02 to 0.16 ppm depending on the product |
| Four straightening treatments per day | Up to 0.44 to 0.75 ppm |
The authors conclude that these treatments, including those labeled "formaldehyde-free", can produce concentrations that reach or exceed occupational exposure limits. The primary exposure is to the hairstylist, who breathes these vapors throughout the day. The client is exposed intermittently, during each session, through the eyes, nose, and scalp, and the acrid odor and stinging eyes during a straightening treatment are the formaldehyde vaporizing.
What straightening does to the scalp and hair fiber
A Mexican dermatology team followed for nearly three years patients who came in for consultation regarding a red, scaly scalp following keratin straightening treatments.
| What was observed | Results |
|---|---|
| Patients | 43, of which 42 women, average age 35 |
| Number of straightening treatments before the reaction | 2 on average |
| Time to onset | 12 months on average after the straightening treatment, shorter when treatments were more frequent |
| Pull test, hair comes out when pulled | Positive in 86% |
| Appearance under dermoscopy | Scales around follicles in 98%, red patches in 81%, onion-skin desquamation in 74% |
The authors conclude that this reaction, which resembles scalp psoriasis, is probably underdiagnosed, because it occurs months after straightening and the connection is not made. The positive pull test in 86% indicates that irritated scalp releases its hair more easily: the hair loss following straightening is first an irritation and breakage, not hair loss from the follicle.
No, itcoatsit. The keratin in the product deposits on the cuticle and superficially fills in damaged scales, which gives the hair a smooth, shiny, soft appearance. But a fiber that has emerged from the scalp is a dead structure that nothing repairs, and exposure to temperatures above 200 degrees, repeated with each straightening, makes it more fragile instead. The effect is real, cosmetic, and temporary: it disappears with shampooing and regrowth, within two to four months, and leaves a fiber slightly more fragile than before.
Oral keratin: the follicle, and two trials
Keratin in capsules does not affect existing hair. A solubilized or hydrolyzed keratin is digested, and its sulfur-containing amino acids, particularly cysteine , are absorbed and transported by the blood to the follicle, which uses them to build the next fiber. Raw keratin, on the other hand, is not digested and serves no purpose; our article on keratin in capsules explains why the form determines everything.
| Trial | Protocol | Result at 90 days, versus placebo |
|---|---|---|
| Cynatine HNS, 50 women, randomized, double-blind, registered | 500 mg per day with vitamins and minerals, versus placebo | Hair loss, growth, resistance significantly improved; amino acid composition of the fiber and shine improved; nails stronger |
| Keratin hydrolysate, women, randomized, double-blind, multicenter | 500 or 1,000 mg per day, versus placebo | Skin, hair shine, nail hardness significantly improved at both doses |
The most telling criterion from the first trial is the amino acid composition of the hair, which changes under keratin: ingested keratin is found in the new fiber. This is exactly the opposite of straightening, which deposits it on top. Both trials include authors affiliated with the manufacturers, which we note as we do for all studies on a patented ingredient.
It does not doesn't smooth. Hair shape—straight, wavy, or curly—is determined by the follicle shape, and no supplement changes it. A better-constructed fiber is shinier, more supple, less breakage-prone, which creates an impression of discipline; but a nourished curly hair remains curly. Expecting a smoothing effect from a capsule is the first misunderstanding in this confusion of names, and it's what makes people say "keratin doesn't work" when they were asking it for something else.
Choose, or combine
If the question is "I want smooth, shiny hair for the next three months," only smoothing answers it, and oral keratin won't change anything. If the question is "I want stronger hair that breaks less, falls out less, and grows better," only oral keratin answers it, because it alone reaches the follicle.
And if both questions apply, which is frequent, the combination is logical : smoothing weakens the fiber through heat and can irritate the scalp; oral keratin strengthens the next fiber and provides the follicle with what it needs. A three-month course started on the day of smoothing prepares stronger hair for the next smoothing treatment. The condition is to choose a smoothing treatment compliant, with a professional who knows what they're applying.
Reading a smoothing treatment label
Since "formaldehyde-free" wasn't sufficient in five products out of six, the ingredient list is the only reference. Names that designate formaldehyde or its precursors:
- Formaldehyde, formalin, formol, methanal, methylene oxide, oxymethylene : formaldehyde itself under its different names.
- Methylene glycol : formaldehyde dissolved in water, the most common precursor in smoothing treatments.
- Glyoxal, glutaraldehyde : neighboring aldehydes, fixatives with similar properties.
- DMDM hydantoin, quaternium-15, imidazolidinyl urea, diazolidinyl urea, bronopol : preservatives that release formaldehyde.
A smoothing treatment that doesn't sting the eyes, doesn't emit an acrid odor when applied with heat, and whose stylist can show the product sheet is a better sign than a mention on the bottle.Glyoxylic acid, the fixative in compliant smoothing treatments, is not a formaldehyde releaser; it lasts less long, and that's normal.
What to do concretely
Frequently asked questions
The difference
What is the difference between keratin in capsules and keratin straightening?
Everything except the name. Keratin straightening is a cosmetic treatment applied to hair already grown: a preparation containing hydrolyzed keratin and a fixative agent is applied to the fiber, then sealed with a flat iron at high temperature; the result is surface-level, immediate and temporary, two to four months. Oral keratin is a dietary supplement: solubilized keratin is digested and its sulfur-containing amino acids reach the follicle that produces the next hair; the result is internal, gradual, three months, and affects the new fiber. One dresses the present hair, the other nourishes the hair to come.
Does oral keratin straighten hair?
No. The shape of hair, straight, wavy, or curly, is determined by the shape of the follicle, and no supplement changes that. Oral keratin makes hair more resistant, shinier, less brittle, which can give an impression of smoothness and discipline, but nourished curly hair remains curly. Expecting a straightening effect from a capsule is the first misunderstanding of this name confusion.
Which one to choose?
They don't answer the same question. If you want straight, shiny hair for the next three months, only straightening does that, and oral keratin won't change anything. If you want more resistant hair, less brittle, that falls out less and grows better, only oral keratin works, because it alone reaches the follicle. Straightening is an immediate aesthetic answer for the present fiber; oral keratin is a deep-acting answer for the fiber to come.
Can you combine both?
Yes, and it's even logical. Straightening weakens the fiber through heat and can irritate the scalp; oral keratin strengthens the following fiber and provides the follicle with what it needs to restart. A three-month treatment that starts when you straighten prepares stronger hair for the next straightening. The only precaution is to choose a compliant straightening treatment, without formaldehyde or precursors, from a professional who knows what they're applying.
Straightening
How does keratin straightening work?
In three steps. A clarifying shampoo opens the cuticle scales. A preparation containing hydrolyzed keratin, conditioning agents and a fixative is applied strand by strand. Then the hair is dried and passed through a flat iron at 200 degrees or more, which causes the fixative to react: it creates bonds between the keratin in the product and that in the fiber, and locks the hair in its straight position. The most effective fixative is formaldehyde or its precursors, which release formaldehyde when heated. It's what makes the straightening last, and it's what causes problems.
Do keratin straightening treatments contain formaldehyde?
Often, even when the label says otherwise. One study measured seven brands of Brazilian straightening sold in South Africa using chromatography: six contained between 0.96 and 1.4% formaldehyde, five times the maximum concentration considered safe, and five of the six were labeled formaldehyde-free. Another study analyzed four professional American products: 11.5% formaldehyde in one, 8.3% in another, 3% in a third, and precursors that degrade to formaldehyde when heated in others. In Europe, formaldehyde has been banned from cosmetics since 2019 and agents that release it must be declared above a threshold, but non-compliant products circulate, particularly online.
Why is formaldehyde a problem?
Because it is classified as a definite human carcinogen by the International Agency for Research on Cancer, and it is irritating and allergenic. During straightening, heat from the blow dryer and flat iron vaporizes it: a salon simulation study measured up to 3.47 parts per million in the air during blow drying, and daily exposures reaching or exceeding professional limits with multiple straightening treatments per day. The hairdresser is the first exposed; the client is exposed sporadically, at each session, through the eyes, nose and scalp.
Can keratin straightening cause hair loss?
It can damage the scalp and weaken the fiber. A study of 43 patients, almost all women, described a scalp reaction appearing on average twelve months after keratin straightening, with redness and scaling, and a positive pull test in 86% of them, a sign that hair detaches more easily. Heat above 200 degrees, repeated, also weakens the fiber, which increases breakage. Hair loss following straightening is therefore primarily breakage and irritation, not loss from the follicle.
Does straightening repair hair?
It coats it. The keratin from the product deposits on the cuticle and fills in the damaged scales on the surface, giving the hair a smooth, shiny and soft feel. But the fiber underneath is not repaired, as hair that has emerged from the scalp is a dead structure that nothing repairs, and passage through the high-temperature flat iron has actually weakened it. The effect is real, cosmetic and temporary: it disappears with fiber renewal and shampooing, in two to four months.
How do you know if straightening is formaldehyde-free?
The label is not enough: five of the six products containing it in the South African study claimed to be formaldehyde-free. Look at the ingredient list and search for precursors: methylene glycol, formalin, methanal, oxymethylene, glyoxal, glutaraldehyde, and releasers such as DMDM hydantoin or quaternium-15. A straightening treatment that doesn't irritate the eyes, that doesn't give off a sharp smell at the flat iron, and whose hairdresser can show you the product sheet is a better sign than a mention on the bottle. In Europe, a compliant product contains none.
Is there a risk-free keratin straightening?
There are straightening treatments compliant with European regulations, formulated without formaldehyde or precursors, with other fixative agents, glyoxylic acid for example. They generally last less long and straighten less radically, which is the price of compliance. They still expose you to heat, which weakens the fiber, and possible scalp irritation. No straightening treatment nourishes the follicle: that's the role of oral keratin.
Oral keratin
How does keratin in capsule form work?
It does not affect existing hair. Solubilized or hydrolyzed keratin is digested, and its sulfur-containing amino acids, especially cysteine, are absorbed and delivered to the follicle, the scalp structure that produces hair. The follicle uses it to build the next fiber, richer in keratin and more resistant. This is why the effect takes three months: it's the time it takes for new hair to grow out and become visible. Raw, untreated keratin does not work because it is not digested.
What do studies show about oral keratin?
Two randomized, double-blind, placebo-controlled trials of 90 days each. The first, in 50 women with Cynatine HNS at 500 mg, shows statistically significant improvements in hair loss, growth, resistance and shine, amino acid composition of the fiber, and nail strength. The second, with a keratin hydrolysate at 500 or 1,000 mg, shows significant improvement in skin, hair shine, and nail hardness compared to placebo. Our dedicated article on keratin capsules details these.
Safety
Is keratin straightening dangerous for health?
The risk comes from formaldehyde, not keratin. Compliant straightening, without formaldehyde or precursor, mainly exposes users to heat and ordinary irritants. Straightening that contains it exposes the client occasionally and the stylist chronically to a definite carcinogen, with air concentrations that reach or exceed occupational exposure limits according to simulation studies. Pregnancy, asthma, reactive skin, and already-irritated scalps are reasons to abstain or demand the product information sheet.
What signs should prompt a consultation after straightening?
A scalp that reddens, itches, or peels in the weeks or months that follow, particularly with flaking around the roots. Increased hair loss, a positive pull test, hair coming out by the dozens when running your hand through. Irritated eyes or throat during the treatment, indicating the product is releasing formaldehyde. In these cases, see a dermatologist and stop straightening treatments pending medical advice.
What should you know about oral keratin and keratin straightening?
That the name is the only thing in common. Straightening deposits a resin on the surface, fixed with heat, often with formaldehyde, including in products labeled without it, with an immediate and temporary cosmetic effect and risk for the scalp and the person applying it. Solubilized oral keratin nourishes the follicle, acts in three months on new fiber, and has two randomized trials to support it. One dresses the existing hair, the other builds the hair to come; they work together, provided the straightening is compliant.
Glossary
- Keratin
- Structural protein of hair, 95% of the fiber, rich in cysteine whose sulfur bonds provide strength.
- Follicle
- Living scalp structure that produces hair; the only possible target of an oral supplement.
- Cuticle
- Outer layer of hair, made of overlapping scales; this is where keratin from straightening is deposited.
- Brazilian straightening, keratin straightening
- Cosmetic treatment depositing hydrolyzed keratin and fixative on the fiber, sealed with a high-temperature iron.
- Formaldehyde
- Irritating and allergenic gas, classified as a definite human carcinogen; effective fixative for certain straightening treatments, banned in cosmetics in Europe.
- Methylene glycol
- Formaldehyde dissolved in water; most common precursor in straightening treatments, which releases formaldehyde when heated.
- Glyoxylic acid
- Fixative for compliant straightening treatments, with no formaldehyde release; lasts shorter.
- Pull test
- Examination consisting of gently pulling on a hair strand; positive when several hairs come out, sign of active hair loss.
- Solubilized keratin
- Keratin whose disulfide bonds have been opened to make it digestible and absorbable; Cynatine HNS is an example.
- ppm
- Parts per million, unit of concentration of a gas in air, used for occupational exposure limits.
Sources
The studies cited below were identified via PubMed.
- Maneli MH, Smith P, Khumalo NP. High concentration of formaldehyde in Brazilian keratin-type straightening products: a cross-sectional study. Journal of the American Academy of Dermatology, 2014;70(2):276-280. DOI
- Pierce JS, et al. Characterization of formaldehyde exposure resulting from the use of four professional smoothing products. Journal of Occupational and Environmental Hygiene, 2011;8(11):686-699. Study conducted by a risk management consulting firm. DOI
- Sanchez-Duenas LE, et al. Psoriasiform cutaneous reaction due to Brazilian keratin smoothing: clinical and dermoscopic study of 43 patients. International Journal of Trichology, 2022;14(3):103-108. DOI
- Beer C, Wood S, Veghte RH. Clinical trial evaluating the effect of Cynatine HNS on hair and nail parameters. The Scientific World Journal, 2014;2014:641723. One author affiliated with the ingredient manufacturer. DOI
- Tursi F, et al. Effects of oral supplementation with natural keratin hydrolysate on skin aging: a randomized, double-blind, placebo-controlled study. Journal of Cosmetic Dermatology, 2025;24(1):e16626. Authors affiliated with the manufacturer and testing laboratory. DOI
