Olaplex vs. K18: Which Is the Better Damage Treatment? I Tested Both to Find Out

ProsSuuuuper fast-acting (4 minutes is all you need)Doesn’t weigh down hairMakes my hair immediately softer and shinierConsA little expensive for a tiny bottleThe first time I tried K18, I immediately understood the hype. I swear I could actually feel my hair become softer over those four minutes, and when it came time to comb, my brush glided through as if I had just used a heavy-duty detangler.I replaced my entire haircare routine with Olaplex a few months back (you can check out the full review here, babes), and don’t get me wrong, I adore how Olaplex made my hair look and feel. However, I saw a much more dramatic difference after using the K18 mask once. My fine, curly hair was much more manageable, less tangled, softer, and bouncier.It’s also tough to beat the short treatment time. In just four minutes, this lightweight mask relinks keratin proteins, addressing damage from deep within and helping it bounce back stronger than it ever was. On days when I use this, I fully skip conditioner and instead smooth one to two pumps (seriously, that’s all I need, and I have looooong hair) through my strands. I then set my timer for four minutes, watch a few TikToks, and comb it through before continuing on with my styling routine.Key ingredient: K18 peptideSize: 1.7 ozUsage: After shampooing, skip conditioner and run one to two pumps through your midlengths and ends, then leave in for four minutes before combing throughOne Cosmo editor says: “I've been using K18 since it launched in 2020, so the results over time have been the most impressive. Truth be told, my hair doesn’t really sustain too much damage now that I have this in my arsenal because of how well it has strengthened my strands.Immediately after I use it, my hair feels extremely soft. Like, shockingly soft. My ends are sealed, my flyaways are smoothed down, and my hair just looks like it did before I started attacking it with box dye, bleach, and flat irons as a teenager.”—BETH GILLETTE, BEAUTY EDITORFeatures at a glanceKey ingredientsK18 peptideSize1.7 ozUsageAfter shampooing, skip conditioner and run one to two pumps through your mid lengths and ends, then leave in for four minutes before combing throughMary HonkusAdvertisement - Continue Reading BelowProsExtremely lightweightGreat for all hair typesMore affordable for a larger bottleConsWorks best in tandem with the rest of the Olaplex lineCan be a bit time-consuming to useThis pre-shampoo treatment does some serious heavy lifting, relinking fractured hair bonds to make hair stronger, healthier, and more resilient. It’s incredibly pleasant to use—I wet my hair, run a palmful through my strands and work my way through unopened emails as it sits for at least 10 minutes (but it doesn’t hurt if you keep it in for longer, btw).In my experience, my hair appears so much shinier and softer with weekly usage. I haven’t minimized heat usage (sorry to my stylist), yet my hair has remained resilient thanks to this.Remember, for the best results, you’ll want to use this along with the Olaplex Bond Building Shampoo, Conditioner, and maybe the Oil (or my favorite, the serum).One Cosmo editor says: “I started using the original Olaplex No. 3 Hair Perfector in 2018, when my hair was severely damaged from heat styling and bleaching. My hair was breaking off, retaining no length. It always looked dull and stringy. Since then, I've used it (and a few other types of bond builders) religiously, and my hair has remained healthy even through multiple color changes and damage from hair extensions. I especially noticed a difference once I started using the whole Olaplex regimen, specifically the leave-in conditioner, compounding those repairing benefits with consistent use in the shower and once my hair was clean too,.”—BGKey features at a glanceKey ingredientsBis-Aminopropyl Diglycol DimaleateSize3.3 oz.UsageOnce a week, apply to damp hair before shampooing and let sit for 10 minutesAdvertisement - Continue Reading BelowFinal verdict:If you’re still debating between the two, our experts recommend thinking about the level of hair damage you’re dealing with. “Olaplex would be best for someone who has minimal chemical damage,” says Goldstein. Think: Split ends and dryness that needs a quick convenient tune up. “K18 is a stronger product, so it would be better for someone with extensive damage from bleaching or constant heat-styling,” she adds.Reminder though: Don’t overdo it. Cardenas says that once your hair feels healthy again, you can cool it on regular application. “Less is more with any product that is changing the molecular level of your hair,” he says. “Too much of it can be damaging to the hair follicle.” With that in mind, if you need light repair, go for Olaplex, but if you have some serious damage, you’re better off with K18.Advertisement - Continue Reading BelowWhy trust Cosmopolitan?Mary Honkus is a contributor for Cosmopolitan with six years of experience researching, writing, and editing beauty stories, including a deep dive on why your hair gets so oily and making a case for coconut oil for hair. She’s an authority in all beauty categories and her hair is her prized possession. For this story, she tested both the Olaplex No. 3 Hair Perfector and K18 Molecular Mask and also asked professional hairstylists which product was better. In the end, K18 came out on top.Mary Honkus is a freelance writer who covers beauty, home, fashion, health, and wellness (just call her a jack of all trades!). She has held staff roles at Real Simple, People, and InStyle, and her bylines can be found in numerous other publications. When she's not testing the latest and greatest products, she can be found chilling on her Hell's Kitchen rooftop, checking out NYC's hottest new restaurants, or recording her podcast Good Trouble NYC. Follow her on Instagram.