Why Are My Nails Brittle? Causes, Tests and the Supplement Studies

Brittle nails that split, peel or break are common, more so in women and with age, and the NHS lists them among ordinary nail changes: it is normal for nails to "become thicker or break more easily (brittle) as you get older", to "become harder, softer or more brittle during pregnancy" and be healthier within six months of having a baby, and most nail problems come from injury, biting, staining, poor trimming, hands often in water or cleaning products, or a fungal infection.¹ A few causes are worth a blood test: iron deficiency anaemia (the NHS lists "nail changes, such as spoon-shaped nails" among its less common symptoms), an underactive thyroid ("dry skin, dry hair or hair loss" on the NHS symptom list) and, rarely, severe zinc deficiency.^{2, 3, 4} Nail psoriasis and fungal infection need a GP, not a supplement.¹ On supplements, the register permits "zinc contributes to the maintenance of normal nails" and "selenium contributes to the maintenance of normal nails"; biotin's nail wording was not authorised after the European assessment of 2010, and every biotin nail study is uncontrolled or retrospective: 41 of 45 evaluable patients improved on 2.5 mg a day in 1989, nail thickness rose 25% in eight patients in 1990, 22 of 35 improved in a 1993 case review, and a 2019 randomised trial added 10 mg a day to a nail lacquer, with 80% against 53% improving by half.^{5, 6, 7, 8, 9} Collagen has one open-label study: 25 people on 2.5 g a day for 24 weeks grew nails 12% faster and broke them 42% less often, with no placebo group.¹³ Fingernails grow about 3.5 mm a month, so any change takes four to six months to reach the tip.¹⁵ The vitamins and minerals guide covers the nutrients.

Evidence at a glance

• NHS nail problems (reviewed 12 March 2024), iron deficiency anaemia (26 January 2024), underactive thyroid symptoms (28 April 2025) and the zinc and selenium lines (3 August 2020, review overdue).^{1, 2, 3, 4}
• Register: zinc and selenium "maintenance of normal nails" authorised (entries 412 and 281); biotin nail wordings non-authorised (entries 2877 and 121), EFSA opinion 2010.⁵
• Five biotin nail studies, 1989 to 2025: one uncontrolled series, one electron-microscopy study in eight patients, one retrospective review, one randomised trial with no biotin-alone arm, one retrospective comparison; none placebo-controlled for biotin alone.^{6, 7, 8, 9, 10}
• Collagen: one open-label study of 25 people (2017) and one placebo-controlled skin trial of 85 women that recorded nail colour (2024), both by supplier-linked authors.^{13, 14}
• Growth rate: fingernails 3.47 mm a month, toenails 1.62, in 22 healthy adults.¹⁵

Sources checked on 1 November 2026.

Why are my nails brittle? The common causes

The NHS's answer is mostly ordinary life. In its words, most nail problems are caused by "injuries or biting your nails", "staining your nails, for example, by smoking or applying a lot of nail varnish", "not regularly trimming your nails, or cutting them at an angle", "your hands often being in water or cleaning products" and "a fungal nail infection".¹ Age is the other large cause: nails become thicker or more brittle as you get older. Pregnancy changes them in either direction, harder or softer or more brittle, and the NHS expects them to be healthier within six months of the birth. A nail that changes colour, loosens and falls off after an injury should grow back within six months on a finger and up to 18 months on a toe.¹ Nails are keratin, the same protein as hair, and the first NHS instruction for hands often in water or cleaning products is rubber gloves.^{1, 6}

What are brittle nails a sign of? When to see a GP

Usually of water, age and knocks. The NHS adds that nail problems "can sometimes be a symptom of a more serious or long-term condition", naming psoriasis, iron deficiency anaemia, an underactive or overactive thyroid, diabetes, and heart, lung or liver disease, and notes that some medicines cause nail problems too.¹ The shapes matter more than the brittleness. Spoon-shaped nails that curve inwards (koilonychia) "can be a sign of iron-deficiency anaemia"; small dents or pits "can be a sign of nail psoriasis, eczema or alopecia"; deep grooves across the nail (Beau's lines) "may happen when you're ill but should grow out"; nails curving over rounded fingertips (clubbing) can be a sign of several serious long-term conditions.¹ The NHS iron deficiency anaemia page lists "nail changes, such as spoon-shaped nails" and "hair loss" among its less common symptoms, and the underactive thyroid page lists "dry skin, dry hair or hair loss" among common ones.^{2, 3} The NHS rule for the GP is plain: see one if "a nail has changed shape, changed colour or fallen off and you do not know why", or if the skin around a nail is sore, red, swollen and warm.¹

Which blood tests matter?

Two, for most people. A full blood count, which the NHS says is the test a GP usually does for suspected iron deficiency anaemia, and a serum ferritin, which measures iron stores and can be low while the blood count is still normal; iron and heavy periods gives the UK thresholds and what is ferritin explains the result.² Thyroid function is the second, because the NHS lists an underactive thyroid among the long-term conditions behind nail problems and "dry skin, dry hair or hair loss" among its symptoms.^{1, 3} A zinc level is only worth asking about in a risk group: a very restricted diet, gut disease, heavy drinking or weight-loss surgery; signs of zinc deficiency sets them out.⁴ One practical point before any of these: high-dose biotin, the amount in many hair, skin and nails formulas, interferes with thyroid and other immunoassays, so tell the clinic and ask how long to stop it beforehand; the MHRA repeated the warning in May 2025.¹⁶

Does biotin help brittle nails? What the studies measured

Every published biotin nail study reports improvement, and none has a placebo group for biotin on its own. The register position follows from that: the European assessment of 2010 did not authorise biotin's nail wordings ("contribute to improve nail's resistance", and "necessary for healthy teeth, bones, hair, skin and nails" for its nail relationship), so only zinc and selenium may say "normal nails" on a label, while biotin keeps its hair and skin wordings.⁵

Two safety facts sit in that table. The 2019 trial used 10 mg of biotin a day, above the NHS line that 0.9 mg or less in supplements is unlikely to cause harm; the 2025 series used 100 mg a day of vitamin B6 in its combined arm, ten times the NHS line of "do not take more than 10mg of vitamin B6 a day in supplements unless advised to by a doctor", because long-term high doses can damage nerves.^{4, 9, 10} A 2018 review of the biotin nail literature reached the same position as this page: firmness, hardness and thickness improved in the studies, and larger controlled trials are needed.¹¹ The systematic review of biotin for hair and nails found 18 published cases of benefit, every one in a person with an underlying cause, and biotin deficiency itself, which is rare, does cause brittle nails along with thinning hair and a scaly rash.¹² So the fair reading is uncontrolled evidence of improvement and a refused claim, both true at once. Is biotin good for hair growth covers the hair evidence, the food amounts and the blood-test rule.

Does collagen make nails stronger?

One study has measured it. In 2017, 25 people with brittle nails took 2.5 g of collagen peptides once a day for 24 weeks and then stopped for four: the growth rate rose by 12%, the frequency of broken nails fell by 42%, 64% were judged improved overall and 88% were still improved four weeks after stopping; there was no placebo group, and the authors work at a collagen research institute.¹³ A second trial, of 85 women aged 43 to 65 randomised to 5 g of collagen peptides or placebo for 84 days with skin as its main question, recorded nail colour improving in the collagen group within 28 days and in the placebo group only by day 84; colour is not strength or growth.¹⁴ Collagen's skin wording is non-authorised and it has no nail entry in the register.⁵ Collagen for hair and nails has the rest.

Zinc, selenium and the white-spot myth

Zinc contributes to the maintenance of normal nails, hair and skin, and selenium to the maintenance of normal nails and hair; those are the two authorised nail wordings, and each may be used only where a daily portion is a source of the mineral.⁵ The NHS lines: "Do not take more than 25mg of zinc supplements a day unless advised to by a doctor", because high doses reduce copper absorption; and 350 micrograms or less a day of selenium in supplements is unlikely to cause harm, while too much "in its mildest form, can lead to loss of hair and nails".⁴ In September 2026 the UK Committee on Toxicity, reviewing selenium in the maternal diet, endorsed in draft minutes the European upper level of 255 micrograms a day from all sources as its guidance value in place of the older 450 microgram expert-group value, noting that NHS advice still rests on that 2003 assessment and that a single high-strength supplement can exceed 255; its final statement had not been published at 1 November 2026.¹⁸ Excess is a real nail problem in its own right: in a 2010 US outbreak, 201 people took a liquid supplement that contained 200 times its labelled selenium, a median of 41,749 micrograms a day, and 61% reported nail discolouration or brittleness; that is a poisoning case, not a comment on supplements at label amounts.¹⁷ White spots on the nails are not a zinc sign: they are almost always small knocks to the nail bed that grew out weeks later, as signs of zinc deficiency explains.

Does calcium make nails strong?

No. Nails are keratin, a protein, not bone, and calcium is not on the NHS list of causes of nail problems.^{1, 6} Brittle nails do occur in hypoparathyroidism, a medical cause of low blood calcium, alongside tingling and cramps, but they are not a sign of a low calcium intake; what are the signs of low calcium separates the two.

What helps brittle nails day to day

The NHS list, in order: wear rubber gloves if your hands are often in water or you regularly use cleaning products; clean nails with a soft nailbrush; apply hand cream to nails and fingertips regularly; trim regularly, which is easier after a shower or bath; cut injured, loose nails back to where they are still attached; do not clean under them with sharp objects; do not bite or pick them or the skin around them; and do not ignore fungal infections on the skin, such as athlete's foot.¹ Then wait. In 22 healthy adults who marked their nails and measured the mark's progress, fingernails grew 3.47 mm a month and toenails 1.62, so a fingernail is replaced in roughly four to six months and a toenail in a year or more; whatever you change, the nail you can see today was made months ago.¹⁵

Who should check first

Anyone who is pregnant or breastfeeding, anyone on levothyroxine or with a thyroid condition, anyone with kidney disease or haemochromatosis, and anyone with sudden, patchy or persistent hair or nail changes should ask their GP or pharmacist first.^{1, 3, 16} Anyone with a blood test booked tells the clinic about biotin.¹⁶ A suspected reaction to a food supplement is reported to your local authority's food safety team, which works with the Food Standards Agency; the Yellow Card scheme is for medicines.

What we don't know

• Whether biotin alone helps brittle nails better than nothing. No placebo-controlled trial of biotin on its own has been published.^{9, 11}
• Whether collagen peptides do. The one nail study had no placebo group.¹³
• How often brittle nails in the UK signal a deficiency. No UK figure exists.
• Literature searched to 1 November 2026.

Frequently asked questions

What vitamin deficiency causes brittle nails?

Iron deficiency is the one the NHS names, through spoon-shaped nails; severe zinc deficiency and biotin deficiency can, and both are rare on a mixed UK diet.^{1, 2, 12} A ferritin test and a full blood count settle the iron question; the symptom alone does not.²

How long does biotin take to work on nails?

In the 1989 series, improvement took an average of 5.5 months, which matches the growth rate: a fingernail is replaced in four to six months.^{6, 15} None of the biotin nail studies had a placebo group, so how much of that change was biotin and how much was time and gloves cannot be said.^{9, 11}

Do white spots on nails mean a deficiency?

No. They are almost always small injuries to the nail bed that grew out weeks later; signs of zinc deficiency lists what zinc deficiency does look like.

Can collagen help nails?

In the one study, 25 people on 2.5 g a day for 24 weeks grew nails 12% faster and broke them 42% less often, with no placebo group; a controlled trial recorded nail colour, not strength.^{13, 14} Collagen has no authorised nail wording.⁵

Sources

1. NHS. Nail problems. Page reviewed 12 March 2024. nhs.uk.
2. NHS. Iron deficiency anaemia. Page reviewed 26 January 2024. nhs.uk.
3. NHS. Underactive thyroid (hypothyroidism): symptoms. Page reviewed 28 April 2025. nhs.uk.
4. NHS. Vitamins and minerals: others (selenium, zinc) and B vitamins and folic acid (vitamin B6, biotin). Pages reviewed 3 August 2020. nhs.uk; nhs.uk.
5. Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. Published 31 December 2020, last updated 19 May 2026. gov.uk. Authorised entries 412 (zinc) and 281 (selenium); non-authorised entries 2877 and 121 (biotin), 1950 (collagen); EFSA Journal 2010;8(10):1728 for the biotin assessment.
6. Floersheim GL. Treatment of brittle fingernails with biotin. Zeitschrift für Hautkrankheiten, 1989. PubMed 2648686. Uncontrolled treatment series, 71 treated, 45 evaluable, 2.5 mg a day for an average 5.5 months; funding not stated on PubMed; German-language paper, English abstract.
7. Colombo VE, Gerber F, Bronhofer M, Floersheim GL. Treatment of brittle fingernails and onychoschizia with biotin: scanning electron microscopy. Journal of the American Academy of Dermatology, 1990. PubMed 2273113. Uncontrolled study with electron microscopy, 8 patients timed with treatment, 14 partly timed, 10 controls with normal nails; funding not stated on PubMed.
8. Hochman LG, Scher RK, Meyerson MS. Brittle nails: response to daily biotin supplementation. Cutis, 1993. PubMed 8477615. Retrospective case review, 35 of 44 patients evaluated over a six-month clinic period; funding not stated on PubMed.
9. Chiavetta A, Mazzurco S, Secolo MP, et al. Treatment of brittle nail with a hydroxypropyl chitosan-based lacquer, alone or in combination with oral biotin: a randomized, assessor-blinded trial. Dermatologic Therapy, 2019. PubMed 31344296. Randomised assessor-blinded trial, 50 adults, 4 months, lacquer alone (26) or with 10 mg oral biotin a day (24); funding not stated on PubMed.
10. Shemer A, Lyakhovitsky A, Hermush V, et al. Biotin or pyridoxine versus combined regimen in the treatment of onychoschizia. Journal of Drugs in Dermatology, 2025. PubMed 39913222. Retrospective comparative study, 61 patients, 3 months of treatment, response assessed at 6 to 9 months; funding not stated on PubMed.
11. Lipner SR, Scher RK. Biotin for the treatment of nail disease: what is the evidence? Journal of Dermatological Treatment, 2018. PubMed 29057689. Narrative review; funding not stated on PubMed.
12. Patel DP, Swink SM, Castelo-Soccio L. A review of the use of biotin for hair loss. Skin Appendage Disorders, 2017. PubMed 28879195. Systematic review of case reports and trials, 18 cases, all with an underlying pathology; funding not stated on PubMed.
13. Hexsel D, Zague V, Schunck M, et al. Journal of Cosmetic Dermatology, 2017. PubMed 28786550. Open-label single-centre study, 25 people, 2.5 g of collagen peptides a day for 24 weeks then 4 weeks off, no placebo group; authors at a collagen research institute linked to the supplier.
14. Vleminckx S, Virgilio N, Asserin J, et al. Influence of collagen peptide supplementation on visible signs of skin and nail health and -aging in an East Asian population: a double blind, randomized, placebo-controlled trial. Journal of Cosmetic Dermatology, 2024. PubMed 39143887. Randomised double-blind placebo-controlled trial, 85 women aged 43 to 65, 5 g a day for 84 days, skin the primary outcome; authors at the peptide maker.
15. Yaemsiri S, Hou N, Slining MM, He K. Growth rate of human fingernails and toenails in healthy American young adults. Journal of the European Academy of Dermatology and Venereology, 2010. PubMed 19744178. Observational measurement study, 22 healthy adults, one to three months; non-US-government research support.
16. MHRA. Safety Roundup, May 2025: be aware of the potential for biotin interference on thyroid function laboratory tests. gov.uk.
17. MacFarquhar JK, Broussard DL, Melstrom P, et al. Acute selenium toxicity associated with a dietary supplement. Archives of Internal Medicine, 2010. PubMed 20142570. Outbreak investigation, 201 cases in 10 US states, median estimated intake 41,749 micrograms a day; US National Institutes of Health funding.
18. Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment. Draft minutes of the 8th September 2026 COT meeting, item 6: selenium in the maternal diet (paper TOX/2026/31). Draft minutes, updated 15 October 2026; a final statement is to follow. gov.uk.

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This article is general information, not medical advice. A nail that has changed shape or colour for no reason you know, or sore, swollen skin around a nail, needs a GP; tell whoever takes your blood if you take high-dose biotin.