Everything You Need To Know About K18, The Celebrity-Beloved Hair Mask

By now you’ve at least heard of K18 – an innovative, fast-acting hair treatment that promises to transform even the most thrashed of strands in just four minutes. Kim Kardashian went on record saying she uses “a lot of K18.” Halsey recently posted the product on her Instagram stories, and other trusted celeb beauty icons like Selena Gomez, Hailey Bieber, and Rihanna have also given the product their blessing.

As if that’s not enough proof, you can trust the swarms of real-life customers who have taken to the Internet to share their love of the cult-favorite hair treatment. The hashtag #k18flip on TikTok has truly taken on a life of its own (currently clocking a cool 11B views). Over on Sephora’s website, you’ll see a flurry of glowing testimonials. “Out of everything I’ve tried for the last 10 years, this one has delivered actual results from first use,” writes one customer. “Wish I could rate this mask more than five stars — I have never experienced such a powerful mask. The results I saw and felt after just one use were incredible,” raves another. With street cred like this, there’s no arguing that the $75 treatment works, but we have questions. What is K18, exactly? How do you use it? What’s in it? And what makes it different from other bond builders on the market?

To sift through the ingredient list and commotion, Bustle asked a celebrity hairstylist and a trichologist (aka a hair and scalp expert) for their no-B.S. insight.

What Is The K18 Hair Treatment?

Think of the treatment as a deeply reparative mask. “K18 is a four-minute leave-on hair repair product that reconnects broken keratin chains which, as a result, re-forms many of the disulfide bonds broken by damage,” explains hairstylist and K18 educator Amber Maynard Bolt. To understand disulfide bonds, think of each individual hair strand as a ladder under a microscope. The vertical sides are called polypeptide chains, and disulfide bonds are the horizontal steps. When there's any kind of trauma to the hair (via bleach, color, chemical services, or heat from hot tools) the disulphide bonds can easily break. Since most people are damaging their strands on the regular, it makes sense that bond-building treatments are more popular than ever.

While the hero ingredient in the K18 hair mask is the brand’s patented peptide, there are others that go to work on your strands. “When looking at ingredients, the first five are the most important, as they usually make up about 97% of a product's volume,” certified trichologist Taylor Rose explains. First up is water, which simply means it’s a water-based rather than an oil-based hair product. Next up is alcohol denat, a generic term for denatured alcohol. “It’s also called ethanol, and can act as a penetration enhancer and is known for its fast drying properties on hair,” says Rose.

Propylene glycol, a highly effective humectant that helps the skin and hair absorb and retain moisture, is third. “It also absorbs excess water in a formula, reducing bacteria growth to help your products last longer,” says Rose. The fourth ingredient is Cetearyl alcohol, an emollient that is used to help soften the skin and hair and to thicken and stabilize products. “It is considered a good ingredient for soothing dry skin and is actually beneficial to the hair and scalp,” Rose explains. And the fifth active is dicaprylyl ether, another ingredient with emollient properties used to soften the hair and help the product spread more easily throughout the hair, says Rose.

What Makes It Different?

One standout perk of K18 is how foolproof it is to use. “You don’t have to rinse out K18 or use a lot of it like traditional bond builders,” says Maynard Bolt. Olaplex No. 3, for example, has to be left on for at least 10 minutes and is meant to be washed out. So for one, K18 is faster and simpler to use. But in terms of its formula, it’s pretty similar to Olaplex’s reparative treatment, says Rose. “The first five ingredients of both products are not that different,” she says. The main contrast between Olaplex No. 3 and the K18 leave-in treatment mask is their unique patented ingredients. “Although, at the end of the day, both formulas claim to rebuild disulfide bonds,” notes Rose.

On a molecular level, Rose explains, the two products re-form the disulfide bonds through different methods. K18 uses the sh-Oligopeptide-78 (K18Peptide™) as its active ingredient, which is said to reconnect broken keratin chains which, as a result, re-forms many of the disulfide bonds broken by heat or chemical damage. Olaplex uses its patented ingredient, Bis-Aminopropyl Diglycol Dimaleate, to find single sulfur hydrogen bonds and cross-link them back together to form disulfide bonds. The trichologist also notes that the No. 3 treatment needs to be done weekly instead of monthly. Since No. 3’s recommended use is more frequent, it may result in more scalp irritation and dry, brittle hair (namely from consistent application of propylene glycol). It’s also worth noting that “Olaplex no. 3 actually has their patented ingredient listed as the second ingredient, meaning it makes up a large portion of the product,” adds Rose.

How To Use K18

Traditional hair masks typically have to sit for five to 30 minutes before you can rinse them out. The fact that you can slap this on and call it a day makes all the difference. You’ll just want to make sure you use K18 after shampooing. “My biggest tip that I give to my clients is to make sure you’re starting with a clean canvas; a detox shampoo will clarify and get rid of any buildup, making it easier for the mask to do what it needs to do,” says Maynard Bolt. Next, skip (yes, skip!) a conditioner. “[Don’t] use conditioner before using this product, as it will coat your strands and not allow the product to penetrate properly,” explains Rose.

Once you’re out of the shower, apply one or more pumps (depending on your hair’s length, thickness, and condition) of K18 on damp strands. Pro tip: Be sure to coat your hair evenly from root to ends. Then, let it sit for those four illustrious minutes before styling as usual. In terms of maintenance, you’re going to want to repeat this process for your next four to six consecutive washes, then rotate it into your regimen monthly (or so, as needed).

Who Can Use K18?

Everyone can benefit from this hair mask, but Maynard Bolt notes that those with more damaged hair see the best results. Also notable? You can use it on extensions. “Not only does it protect the integrity of the hair, but it also helps with length retention, and maintains the hair’s softness and bounce,” says Maynard Bolt. Rose, however, does note that because of the presence of alcohol denat, it can sometimes have a drying effect on curly hair types. “[However], there is no scientific evidence to suggest that hairstyling products containing ethanol dry out the hair permanently or damage it,” says Rose.

According to Maynard Bolt, the very minimal amount of alcohol serves as an agent to optimize the hair environment for the K18 peptide to work on a deeper level and not to dry hair out. As for propylene glycol, Rose says it can cause irritation or inflammation in your scalp with prolonged use. So if you use the ingredient too often, you might experience dry, brittle strands, which are the most common propylene glycol side effects for hair. Pro tip: Don’t use it more often than directed.

TL;DR

In the end, $75 is certainly on the pricier side for a mask. However, it’s anything but overly hyped — all signs point to shinier, healthier hair.

Studies referenced:

Final report of the safety assessment of Alcohol Denat., including SD Alcohol 3-A, SD Alcohol 30, SD Alcohol 39, SD Alcohol 39-B, SD Alcohol 39-C, SD Alcohol 40, SD Alcohol 40-B, and SD Alcohol 40-C, and the denaturants, Quassin, Brucine Sulfate/Brucine, and Denatonium Benzoate. Cosmetic Ingredient Review, Washington DC 20036, USA. DOI: 10.1080/10915810802032388

Fraser, R.D. (2012). The role of disulfide bond formation in the structural transition observed in the intermediate filaments of developing hair. J Struct Biol. https://pubmed.ncbi.nlm.nih.gov/22683767/

Ha, E. J., Yun. (2020). Application of Ethanol Extracts From Alnus sibirica Fisch. ex Turcz in Hair Growth Promotion. Frontiers in Bioengineering and Biotechnology, 9. https://doi.org/10.3389/fbioe.2021.673314

Tata, S. (1994). Relative influence of ethanol and propylene glycol cosolvents on deposition of minoxidil into the skin. J Pharm Sci. https://pubmed.ncbi.nlm.nih.gov/7884676/

Yang, C. (2013). The structure of people’s hair. PeerJ, 2. https://doi.org/10.7717/peerj.619

Experts:

Amber Maynard Bolt, hairstylist and K18 educator

Taylor Rose, certified trichologist