Keratin and protein treatments: do they help thinning hair?

"Keratin" is one of the most common words in hair care marketing. It appears on shampoos, conditioners, masks, leave-in sprays and, most prominently, on salon smoothing services sold as "keratin treatments" or "Brazilian" treatments. Protein treatments and masks are often recommended for hair that feels weak or breaks easily. For someone whose hair is thinning, the question is simple: can putting keratin or protein on hair make thinning hair better? This guide explains what keratin and protein products do, what they cannot do, and the safety issues with some salon smoothing treatments, including formaldehyde and a more recently recognised kidney risk from glyoxylic acid.

(This is general information, not personal medical advice.)

Short answer

Hair is made mainly of keratin protein ¹, so keratin and protein products work on the hair shaft, the part of the hair above the skin. Laboratory work shows protein hydrolysates can deposit on the hair fibre and improve hydration, shine and softness ¹. That can make damaged hair look and feel better and may reduce breakage, but it does not change the hair follicle, so it does not treat thinning caused by pattern hair loss, shedding or scalp disease. Salon smoothing treatments carry real risks. Many contain formaldehyde-related ingredients that release formaldehyde gas when heated ². Products with glyoxylic acid have been linked to acute kidney injury ³. Brazilian keratin treatments have also been linked to a scaly scalp reaction with increased hair shedding ⁴. If your hair is thinning, find the cause first.

What keratin is, and why hair products use it

Keratin is the main structural protein of hair, alongside a small amount of lipid ¹. Each hair has an outer layer of overlapping cells called the cuticle, which protects the inner cortex. Washing, brushing, sun, heat styling, colouring and chemical straightening can lift and wear away the cuticle and weaken the fibre. A review of hair cosmetics notes that shampoos, dyes and straightening products can alter hair fibre structure, remove lipids and strip protein ⁵.

Protein products aim to patch up some of that damage on the surface. They do not add living keratin to the hair. Hair above the scalp is not alive and cannot rebuild itself. What a product can do is coat and fill in parts of the fibre so it feels smoother and holds together better.

Three very different things sold as "keratin"

1. Protein shampoos, conditioners and masks

These products contain hydrolysed proteins, which are proteins broken into smaller fragments. They may come from keratin (from wool, feathers or horns), wheat, silk, rice, soy or milk. In one laboratory study, a mild shampoo and a rinse-off conditioner made with keratin peptides were applied to hair fibres. Hydration improved, more so with heat, and electron microscopy showed deposits at the junctions of the cuticle cells, which probably helped seal the cuticle and increase shine and softness ¹.

This was a laboratory study on hair fibres, not a trial in people with thinning hair. It supports the idea that protein products can improve how damaged hair feels and looks. It says nothing about hair growth.

2. Salon keratin smoothing treatments

These are the "keratin treatment", "keratin blowout" or "Brazilian" services offered by many salons. A smoothing solution is applied to the hair and then heat, usually from a flat iron, seals it in. The result is smoother, less frizzy hair for weeks to months.

Despite the name, keratin is not what does most of the smoothing. The United States Food and Drug Administration (FDA) explains that hair smoothing products often contain formaldehyde, also called formalin or methylene glycol, and that when the solution is heated, formaldehyde is released into the air as a gas ². Products marketed as "formaldehyde-free" may use other chemicals, including glyoxylic acid ⁶.

3. Rebonding, relaxers and Japanese straightening

Japanese straightening, rebonding and relaxers are different again. They chemically break and re-form bonds in the hair to change its shape. They are common in Malaysia and carry their own risk of breakage and scalp injury. See Hair fall after rebonding: breakage or real hair loss? and Chemical straightening and the scalp: what studies report.

Can keratin or protein treatments help thinning hair?

It depends on what "thinning" means.

If hair is breaking. Hair that snaps along its length, splits at the ends or feels rough after bleaching or heat styling can look thinner because the ends are uneven and wispy. Protein and conditioning products may help such hair look smoother and resist breakage from combing. Reducing the damaging habits matters more. See Hair breakage vs hair loss: how to tell.

If hair is falling from the root, or the scalp is showing more. This is a follicle problem: pattern hair loss, telogen effluvium, alopecia areata, a scalp condition, or another medical cause. A product on the hair shaft does not change what happens in the follicle. Keratin treatments do not treat these conditions.

The look of smoothed hair. Smoothing treatments make hair sleeker and flatter. For some people with fine or thinning hair, flatter hair can make the scalp more visible, so the result may look thinner rather than fuller. Others like that smoother hair tangles less. It is a cosmetic trade-off worth discussing with a stylist.

A study of 94 young Indian women found that those who had blow-dried, straightened, ironed or permed their hair in the previous six months were far more likely to show microscopic changes in the hair shaft than women who had not, although hair density and a breakage index were not significantly different ⁷. The hair may look fine to the eye while the fibre is already damaged.

Safety: what to know before a salon smoothing treatment

Formaldehyde

The FDA describes formaldehyde as a colourless, strong-smelling gas that is a health hazard when breathed in or when it gets into the eyes or onto the skin. Citing the National Cancer Institute, it notes that at air levels above 0.1 ppm some people may have watery eyes, burning in the eyes, nose and throat, coughing, wheezing, nausea and skin irritation. Some people are very sensitive and others have no reaction to the same exposure. Repeated contact with formaldehyde-related ingredients can cause skin sensitisation or allergic dermatitis. Formaldehyde has been classified as a human carcinogen by the International Agency for Research on Cancer because high or prolonged exposure has been linked to certain cancers. If a salon is poorly ventilated, both the staff and the clients are at risk of breathing in the released gas. The FDA advises consumers to look for formaldehyde, formalin or methylene glycol on labels and to ask their salon whether the product contains them ². Salon workers are exposed again and again, which is worth keeping in mind if you work in a salon.

Glyoxylic acid and the kidneys

Glyoxylic acid is used in some "formaldehyde-free" straightening products. In 2024, French doctors reported in the New England Journal of Medicine on kidney injury linked to hair-straightening products containing glyoxylic acid ⁶. A follow-up paper described a patient who had three separate episodes of acute kidney injury, each after a "Brazilian" hair-straightening treatment with a glyoxylic acid cream. In mice, the straightening product and a glyoxylic acid cream applied to the skin were absorbed, sharply raised urine oxalate and caused calcium oxalate deposits in the kidneys. The authors concluded that cosmetic products containing glyoxylic acid may cause acute kidney injury and should be discontinued ³.

If you develop nausea, vomiting, back or flank pain, or pass less urine in the days after a straightening treatment, seek medical care and tell the doctor about the treatment.

Scalp reactions and shedding

A study from Mexico of 43 people, nearly all women, who developed redness and scaling on the scalp after Brazilian keratin treatments found that the hair pull test was positive in 86%, meaning hairs came out easily. The most frequent findings on dermoscopy were scales around the follicles and red patches. On average the patients had had about two treatments, and the reaction appeared about a year later, although this varied widely and was related to the number of treatments. The authors suggest this reaction may be underdiagnosed ⁴.

A systematic review of chemical straighteners found that in some studies they were associated with eczema, scaling, pain, burns and inflammation of the scalp, as well as hair loss, damage to the shaft and changes in hair colour. Long-term effects remain unknown ⁸.

Pregnancy and sensitive scalps

A review of hair cosmetics notes that dyeing and straightening procedures have side effects and health concerns, especially for pregnant women and people with sensitive hair and scalp ⁵. If you are pregnant, consider postponing chemical smoothing and ask your doctor.

Using protein products sensibly

• Match the product to the problem. Protein masks are for damaged, breaking hair, not for hair falling from the root.
• Use conditioner regularly. Conditioners are designed to reduce friction and tangling between hairs ⁹.
• Reduce the cause of damage. Less frequent bleaching, lower heat settings and gentle detangling usually matter more than any single product. See Heat styling and hair damage: safer habits.
• Be wary of "repair" claims. A product can make hair look and feel better, but damaged hair is only truly replaced as new hair grows and the old ends are trimmed.
• Watch for scalp reactions. Stop any product that makes your scalp itchy, red or sore.
• Keep a record. Note the date of each salon treatment, the product name and any reaction afterwards, so a doctor or stylist can see a pattern if problems recur. Monthly scalp photos in the same light also help.

Questions to ask before a salon smoothing treatment

• Does the product contain formaldehyde, formalin, methylene glycol or glyoxylic acid?
• Can I see the ingredient list or safety data sheet?
• How is the room ventilated?
• How much heat will be used, and how often should it be repeated?
• What should I do if my scalp becomes red, itchy or sore afterwards?

If the answers are vague, you can decline.

When to see a doctor

See a doctor if:

• hair is falling out from the root in larger amounts than usual, or in patches
• your part is widening or more scalp is showing
• the scalp is red, scaly, burning or painful after a hair treatment
• you feel unwell after a straightening treatment, especially with nausea, vomiting, back pain or less urine
• you are unsure whether you have breakage or true hair loss

Life in Malaysia

Smoothing, rebonding and keratin services are widely available in Malaysian salons, and Malaysia's humidity makes frizz a common reason people choose them. Treatments and products vary, and labels or ingredient lists are not always easy to see in a salon. Ask before the treatment starts, and avoid repeat treatments close together. Cosmetic products sold in Malaysia must be notified to the National Pharmaceutical Regulatory Agency (NPRA) ¹⁰, but notification is not an approval that a product treats hair loss. If you wear a hijab, give treated hair time to dry fully before covering it. For a gentle routine, see A gentle hair care routine when you are losing hair and How to make thin hair look fuller: styling tips. If you are buying products from a pharmacy, see Hair loss products at the pharmacy: what to ask the pharmacist. If you see a doctor, choose a registered practitioner; you can check registration with the Malaysian Medical Council ¹¹. For an overview of treatment options, see Hair loss treatment in Malaysia: a plain guide to your options.

Bottom line

Keratin and protein products act on the hair shaft. They can make damaged hair smoother, shinier and possibly less prone to breakage, which helps if your "thinning" is really breakage. They do not treat hair loss from the follicle. Salon smoothing treatments are a separate matter: many release formaldehyde when heated, glyoxylic acid products have been linked to acute kidney injury, and Brazilian keratin treatments have been linked to scaly scalp reactions with shedding. If hair is thinning, get the cause checked, and treat smoothing as a cosmetic choice with known risks.

Frequently asked questions

Do keratin treatments help thinning hair?

They can make damaged hair smoother and may reduce breakage, but they act on the hair shaft and do not treat hair loss from the follicle, such as pattern hair loss or shedding.

What do protein masks actually do?

Hydrolysed proteins can deposit on the hair fibre and improve hydration, shine and softness. The evidence comes mainly from laboratory studies on hair fibres.

Is a salon keratin treatment the same as a keratin shampoo?

No. Salon smoothing treatments use chemical solutions sealed with heat, and many contain formaldehyde-related ingredients. Keratin shampoos are rinse-off conditioning products.

Why is formaldehyde a concern in smoothing treatments?

When the solution is heated, formaldehyde is released as a gas that can irritate the eyes, nose, throat and skin. It is classified as a human carcinogen, and repeated contact can cause skin sensitisation.

Are formaldehyde-free treatments safe?

Not necessarily. Some use glyoxylic acid, which has been linked to acute kidney injury after hair-straightening treatments. Ask what the product contains.

Can a Brazilian keratin treatment cause hair loss?

A study of 43 patients with a scaly scalp reaction after Brazilian keratin treatment found a positive hair pull test in 86%. Chemical straighteners have also been linked to scalp burns and hair loss.

Can smoothing make thin hair look thinner?

It can for some people, because flatter, sleeker hair may show more scalp. Discuss the trade-off with a stylist.

When should I see a doctor?

See a doctor if hair is falling from the root, a part is widening, the scalp is red or painful after a treatment, or you feel unwell with nausea, vomiting, back pain or less urine after straightening.

Educational content. Not medical advice.