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It takes a truly effective product to reach holy grail status in a few short years. K18’s Leave-In Molecular Repair Hair Mask did just that, emerging in late 2020 as the new ‘It’ girl of bond-builders.
The mask works as a leave-in treatment, designed to reverse damage from bleach, color, chemical services, and heat in four minutes flat. It’s formulated with a proprietary biomimetic ingredient, K18Peptide, which reconnects broken polypeptide chains and disulfide bonds. In non-science terms, this treatment makes your hair look very good very quickly.
The K18 Leave-in Molecular Repair Hair Mask works a little differently than the OG bond-builder, Olaplex. K18’s peptides bind to damaged keratin proteins to repair them while Olaplex focuses on forming new bonds, so the two can be combined if desired. Technically speaking, K18 goes deeper to repair the damage, but both can help improve hair quality.
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In addition to K18 leave-in hair mask’s repairing benefits, it also speeds up your routine, as there is no need to condition after shampooing or step out of the shower to let it set before rinsing. It might seem counterintuitive to skip conditioner, but this actually allows the mask to penetrate the hair shaft more effectively. After waiting four minutes, you can apply additional leave-in conditioners or hair styling products as desired.
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Our beauty writer was introduced to the K18 Molecular Repair Hair Mask in 2022 and has been using it religiously ever since. Read on for her honest, unbiased review and see her impressive before and after photos.
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How We Tested the K18 Hair Mask
- Product tested: K18 Leave-In Molecular Repair Hair Mask
- Number of testers: 1
- Testing time period: 2 years
- Key ingredients: K18Peptide, which is clinically proven to repair damage from bleach, color, chemical services, and heat;
- Price: $29 for 15ml; $75 for 50ml
- Pros:
- Reverses damage in just four minutes
- Restores strength, softness, smoothness and bounce
- Gives both immediate and long-term results
- Makes hair routine quicker
- Cons:
- Can be costly if used often
K18 Leave-in Molecular Repair Hair Mask
Price upon publish date of this article: $75
K18 Hair Mask Review — Testing Notes
Tldr; K18’s leave-in hair mask is actually magic in a bottle.
As a seasoned reporter on the beauty industry, I’m typically wary of anything with claims that sound too good to be true, but K18 truly lives up to the hype. The brand’s Leave-In Molecular Repair Hair Mask literally saved my strands after a manic, COVID-era coloring spree. Not-so-fun fact: Years of beauty expertise can fly out the window during a mental health episode. I’ve since found mood stabilizers and K18, so my hair and I are much healthier now.
I first tested the K18 hair mask two years ago after a recommendation from celebrity colorist Rachel Bodt, who did my hair before COVID kept me away from the salon. (Rachel also does Beyoncé’s hair, so I’d trust her with my life.)
Before K18, my color was shifting as quickly as my mood. I went from bright red to pastel pink in a few months, bleaching in between with minimal damage. I used a demi-permanent color for the red and color-depositing conditioner for the pink, both of which faded gradually and didn’t require much of a lift in between.
It wasn’t until I tried a “ginger” colored conditioner that things really went south. The shade was more orange than copper as I’d expected, but I decided to roll with it for a bit. I assumed it would fade out like the others, but I was very, very wrong. It stained my hair cuticle in a way I’d never seen before in all of my years of testing hair products. (Turns out, I was just one of many victims of the ginger stain.)
In a desperate attempt to get the orange out, I bleached it again, but that barely removed any pigment. I then tried Color Oops Hair Color Remover, dandruff shampoo, dish soap, and any other dye-stripper you can think of. (Pro tip: Do not do this.) The orange went away after about three weeks, but my hair was absolutely wrecked, extremely porous, and completely lacking moisture. It turned to mush when wet. Any of my natural wave was gone, replaced by a frizzy, fried texture. It looked voluminous in a comical way because my cuticle had been so roughed up that my strands were puffy. Some pieces of it started to break off, too. Not my best look.
After I started using K18, I could not believe how quickly it worked to repair the damage. My straw-like strands felt silkier and looked shinier after just one treatment. One! So, they weren’t kidding about that whole “reverses damage in four minutes” thing. I was hooked immediately.
Technically, you only need to use it four to six times following a salon service and then as needed. Well, I needed it, so I used it every day for a month to start. In that time, I noticed a significant improvement in both the strength and softness of my hair.
After the first month, I got a few highlights to break up my harsh root line instead of double processing as I had been. I went down to using K18 twice a week for the next two months. By that point my hair had completely transformed, even with the additional chemical process.
During these first few months of using it, I did not use any treatments other than K18. I shampooed with Pura D’Or Gold Label Shampoo four to five times a week and about once a week with Kérastase Blond Absolu Purple Shampoo to combat brass.
I’ve since weaned off the bleach and returned to a more natural red to give my hair a true break. Now that it’s much healthier, I use K18 a few times a month for maintenance versus severe damage control. The more I use it, the better my hair looks. K18’s hair mask can get pretty costly if you use it often, but in my book, it’s worth every penny.
K18 Leave-in Molecular Repair Hair Mask
Price upon publish date of this article: $75
More K18 Hair Care Products to Shop
K18 Molecular Repair Hair Oil
Price upon publish date of this article: $65
K18 Damage Repair Essentials Kit
Price upon publish date of this article: $58
K18 AirWash Dry Shampoo
Price upon publish date of this article: $48
K18 Damage Shield Protective Shampoo
Price upon publish date of this article: $25.65
K18 Damage Shield Protective Conditioner
Price upon publish date of this article: $27
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Meet the Author
Katie McBroom is an award-winning writer and content creator with over a decade of experience in digital media. Prior to contributing to WWD, she served as Content Editor for Google and Beauty Editor for Hearst’s Best Products. Her work has also appeared in publications including Cosmopolitan, Elle, Oprah, CNN, Business Insider, and Men’s Health, among others. Well-versed in all things beauty, she is also a licensed cosmetologist and professionally certified makeup artist. McBroom has been using K18 for over two years, and she used her firsthand experience to write this honest review.