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Personal care · product guide

Hair treatment & mask: does it work?

Masks, bond repair and scalp serums condition damaged hair. For thinning, only minoxidil (and prescription options) has strong trial evidence.

Moderate evidenceReviewed September 2026

What it is

This shelf mixes two different kinds of product. Hair-shaft treatments (masks, bond-repair treatments, leave-in serums and oils) aim at bleached, heat-damaged or dry hair. They are intensive conditioners: they coat and smooth, and bond-building products aim to re-link some broken bonds inside the shaft. They change how hair looks, feels and breaks, not how it grows.

Scalp and regrowth treatments aim at thinning. Here the evidence is narrow: topical minoxidil is the non-prescription first-line option for pattern hair loss in men and women, and oral finasteride is a prescription option for men. Most 'growth serums' use caffeine, biotin, peptides or plant extracts without controlled evidence; rosemary oil has one small trial. Name the type of hair loss first; the hair lesson covers how.

How strong is the evidence?

Moderate evidence

For regrowth, the evidence is strong but narrow: 5% topical minoxidil beat 2% and placebo in a 48-week trial of 393 men, and finasteride increased hair counts over two years in 1,553 men. The rosemary-oil claim rests on one small trial against 2% minoxidil with no placebo. For masks and bond repair, independent trials are largely absent; benefits are cosmetic and measured in the lab. A large cohort linked frequent chemical straightener use with higher uterine cancer rates, an observational finding.

The claims you'll see, checked

Minoxidil regrows hair in pattern hair loss

Holds up

Randomized, placebo-controlled trials in men and women; 5% outperformed 2%. Gains fade within months of stopping.

Growth serums with caffeine, biotin or peptides regrow hair

Unproven

Lab and small company studies only. Biotin helps only if you are deficient, which is rare.

Rosemary oil works as well as minoxidil

Mixed

One 100-person trial found similar hair counts to 2% minoxidil at six months, with no placebo group and against the weaker strength.

Masks and bond builders repair damaged hair

Mixed

They improve feel, shine and breakage in lab tests; damaged hair still can't regenerate, and independent trials are scarce.

Keratin smoothing treatments are harmless

False

Some release formaldehyde when heated with a flat iron, a known carcinogen when inhaled at high exposures, and FDA has warned about them.

What to look for

  • For thinning: topical minoxidil, 5% foam or solution for men; for women, the 2% solution or 5% foam sold for women, used as labeled.
  • For bleached or heat-damaged hair: a rich mask or bond-building treatment used weekly, plus less heat and gentler handling.
  • Ingredient lists without formaldehyde, formalin, methylene glycol or glyoxylic-acid 'smoothing' chemistry if you want to avoid formaldehyde-releasing treatments.
  • Realistic timing: minoxidil needs about 4 months for first results and 6 to 12 months for a fair verdict.

What to skip

  • Hair gummies, biotin and collagen for growth unless a test shows you are deficient.
  • Growth serums and 'follicle activators' priced like medicine without trials.
  • Scalp 'detox' treatments.
  • Stacking several masks and oils; one conditioning treatment a week is enough.

How to use it

  • Minoxidil: apply to a dry scalp twice daily (solution) or as labeled (foam), and keep going; the benefit lasts only as long as you use it.
  • Masks: apply to clean, damp mid-lengths and ends for the labeled time, then rinse.

Who should be careful

  • Don't use minoxidil while pregnant or breastfeeding. Expect a temporary increase in shedding in the first weeks; it can cause scalp itch and unwanted facial hair, and is toxic to cats if they lick treated skin or bedding.
  • Finasteride and oral minoxidil are prescription drugs; use them only under a clinician's care, including through telehealth services.
  • Formaldehyde-releasing smoothing treatments can cause eye, nose and throat irritation; avoid them, especially in pregnancy, and ask salons what they use.
  • Frequent use of chemical straighteners and relaxers has been linked with uterine cancer in a large cohort; it is observational, but a reason to use them less.
  • See a dermatologist for sudden, patchy or rapid hair loss, or a painful, red or scaly scalp; blood tests for iron and thyroid may be worth doing.

The studies

Every citation is checked against its PubMed record.

  1. A randomized clinical trial of 5% topical minoxidil versus 2% topical minoxidil and placebo in the treatment of androgenetic alopecia in men
    Olsen EA, Dunlap FE, Funicella T, Koperski JA, Swinehart JM, Tschen EH, Trancik RJ · Journal of the American Academy of Dermatology · 2002 · RCT (placebo-controlled)

    393 men with pattern hair loss used 5% minoxidil, 2% minoxidil or placebo twice daily for 48 weeks. 5% was superior to both, with 45% more regrowth than 2% and an earlier response, but more itching and irritation.

  2. Finasteride in the treatment of men with androgenetic alopecia. Finasteride Male Pattern Hair Loss Study Group
    Kaufman KD, Olsen EA, Whiting D, Savin R, DeVillez R, Bergfeld W, Price VH, Van Neste D, Roberts JL, Hordinsky M, Shapiro J, Binkowitz B, Gormley GJ · Journal of the American Academy of Dermatology · 1998 · RCTs (placebo-controlled)

    In two one-year trials of 1,553 men, finasteride 1 mg daily increased hair counts versus placebo (107 hairs at 1 year and 138 at 2 years in a 1-inch circle), while placebo users kept losing hair. The trials were run by the manufacturer.

  3. Rosemary oil vs minoxidil 2% for the treatment of androgenetic alopecia: a randomized comparative trial
    Panahi Y, Taghizadeh M, Marzony ET, Sahebkar A · Skinmed · 2015 · RCT (active comparator, no placebo)

    100 people with pattern hair loss used rosemary oil or 2% minoxidil for six months. Both groups had more hairs at six months with no difference between them; scalp itching was more frequent with minoxidil.

  4. Use of Straighteners and Other Hair Products and Incident Uterine Cancer
    Chang CJ, O'Brien KM, Keil AP, Gaston SA, Jackson CL, Sandler DP, White AJ · Journal of the National Cancer Institute · 2022 · Prospective cohort

    Among 33,947 women followed for about 11 years, using hair straighteners in the prior year was associated with higher uterine cancer rates (HR 1.80), and frequent use more so (HR 2.55). Dyes and perms were not associated.