Citable summary
Citable summary of this review
K18 Leave-In Molecular Repair Hair Mask is a four-minute, no-rinse treatment built on patented K18PEPTIDE for cosmetic repair of bleach, colour, chemical and heat damage as claimed by the brand. The Cast Index assigns it to Hair as silhouette as leave-in maintenance that keeps casting length workable between salon services.
Quote with attribution to The Cast Index Beauty Desk, reviewed 2026-07-28.
01 / The basics
What is Leave-In Molecular Repair Hair Mask?
K18 describes a leave-in mask with patented K18PEPTIDE that the brand says reverses damage from bleach, colour, chemical services and heat in four minutes; shampoo without conditioner, towel-dry, apply, wait four minutes, do not rinse; vegan, colour-safe and cruelty-free per brand.
- Best for
- Post-chemical and heat-damage maintenance
02 / Review
What is in Leave-In Molecular Repair Hair Mask?
K18’s Leave-In Molecular Repair Hair Mask is built around K18PEPTIDE, a biotech peptide the brand says reconnects broken polypeptide chains and supports disulfide bonds damaged by bleach, colour, chemical services and heat. It is leave-in: the peptide stays; you do not rinse after the four-minute window.
Brand language uses “repair” and “reverse damage.” Treat those as manufacturer cosmetic claims. The Cast Index does not present K18 as a medical or laboratory service. Strength and elasticity percentages published with mist-plus-mask bleaching protocols are omitted from the catalogue fact to avoid overstating a system study.
03 / Review
What should you know about season finish — hair as silhouette?
Hair as silhouette needs length that still holds a line — soft volume, wet-slick, or deliberate undoing. Chemically worked hair frays that line. K18 is the between-salon maintenance mask that keeps texture workable so the silhouette remains a casting decision, not a damage distraction.
Use with Dream Coat when the week needs both internal soft strength and external humidity polish — mask on wash day, Dream Coat on the sleek-set day. Do not collapse both into one Amazon card note.
04 / Review
What should you know about working kit?
In shower: shampoo only — skip conditioner before K18. Out of shower: towel-dry thoroughly. Start with one pump, add by length and density, ends toward roots. Wait a full four minutes. Do not rinse. Then comb and style; add leave-in conditioner only after the window if still needed. Brand suggests intensive use for the first 4–6 washes post-service, then every few washes.
Wrong if the wearer insists on conditioning before the mask — K18 says that blocks the method. Wrong as a daily heavy cream on fine hair without dosage control. Differentiate from Olaplex No.3: No.3 is a pre-shampoo treatment; K18 is a post-shampoo leave-in.
05 / Review
What should you know about limitations and brand cautions?
Contains fragrance allergens on the INCI (e.g. geraniol, linalool, hexyl cinnamal) — check pack if sensitive. Keep out of eyes. Results vary by hair history. Clarifying shampoo first if product build-up is heavy, per brand do’s. Not a substitute for professional assessment of severely compromised hair.
06 / How to use it
How should you use Leave-In Molecular Repair Hair Mask?
- 01Shampoo; do not apply conditioner before the mask
- 02Towel-dry thoroughly
- 03Apply 1–3 pumps from ends toward roots, adding by length and density
- 04Leave for 4 minutes; do not rinse
- 05Comb and style as usual
07 / Quick facts
What should you know before choosing Leave-In Molecular Repair Hair Mask?
- —Leave-in mask; patented K18PEPTIDE (sh-Oligopeptide-78 on INCI)
- —Brand: four-minute leave-in reverse of damage from bleach, colour, chemical services and heat
- —Method: shampoo, do not condition; towel-dry; 1–3 pumps ends-to-roots; wait 4 minutes; do not rinse
- —Vegan, colour-safe, cruelty-free per K18
- —Brand clinical percentages that used mist + mask in bleaching protocols are system results — not restated here as solo-mask guarantees
- —Different job from Olaplex No.3 (pre-shampoo bond treatment) — keep SEO format-specific
