Olaplex vs. K18: Which Is the Better Damage Treatment? I Tested Both to Find Out
The battle for healthy hair is on.💥🥊
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Split ends, dryness, heat damage, brittleness—chances are everyone reading this struggles with at least one of these hair woes. If you’re anything like me, you’ve dealt with all of them. I used to spend major $$$ at the salon to keep my mane as healthy as possible, but since bond-building treatments have become so popular, I’ve been able to restore my hair to its virgin glory. There’s no shortage of bond-building treatments, but two are essentially the category’s poster children: Olaplex and K18.
Olaplex completely changed the hair scene forever once it debuted in 2014. It featured a groundbreaking proprietary ingredient called Bis-Aminopropyl Diglycol Dimaleate that helps mend damaged hair strands, and since then, its bestselling, mega-viral No. 3 Hair Perfector Treatment has become legendary in its own right.
Compared to Olaplex, the K18 Leave-In Molecular Repair Hair Mask is still relatively new to the game (it launched in 2020). That said, it has quickly risen in the ranks to become a favorite reparative treatment of professional stylists and hair enthusiasts alike for its fast-acting, deeply nourishing formula.
Both products repair damaged hair, but does one perform better than the other? After testing out both and asking the pros for their take, we have declared a winner—and the product that comes out on top may surprise you.
What makes Olaplex a good hair treatment?
“Olaplex works to repair broken disulfide bonds, which are susceptible to damage from bleach, color, and heat,” explains hairstylist Jessica Goldstein. The key lies within the patented molecule Bis-Aminopropyl Diglycol Dimaleate (yes, it’s a mouthful—don’t worry about figuring out how to pronounce it), which is featured in all of the brand’s products. A difference in hair can be seen and felt after just one use, though it’s recommended to stay consistent and incorporate the full line of products for the best results.
What makes K18 a good hair treatment?
K18 has made a name for itself thanks to its fast-acting reparative mask that’s said to mend hair in four minutes flat. Using the patented K18 peptide complex, this treatment can reverse damage from color, bleach, heat, and chemical services (aka pretty much anything that makes hair brittle and weak). It does this by targeting keratin, the building blocks of hair. “This technology aims to repair the protein structure, addressing deeper damage and giving hair both strength and elasticity by reconnecting polypeptide chains with keratin,” explains Goldstein.
While many people tend to skip over hair masks because they can be tedious to use (who has 10 to 20 minutes to sit around in the shower? Not me!), K18 easily fits into busy schedules. It doesn’t need to be rinsed out—just combed through damp, clean strands—and then left to sit on hair for just four minutes before styling or layering on other products. Celebrity hairstylist Jae-Manuel Cardenas has also found the lightweight texture to be ideal for all hair types and textures, even curly and fine hair.
What are the main differences between Olaplex and K18?
At their core, both products work to repair damaged hair. However, both do so a little differently. First off, the Olaplex No. 3 Bond Perfector is a rinse-out treatment that should be applied before shampooing. It needs to sit on hair for a minimum of 10 minutes prior to washing your hair. K18, on the other hand, goes on after washing your hair as a leave-on treatment.
Another major difference that both our experts point out? While you can use the Bond Perfector on its own, Olaplex works best as a system and it’s recommended that you incorporate an additional two to three products into your routine (like the shampoo, conditioner, treatment, and oil) to experience the most significant results.
Meanwhile, “K18 is simpler with a single leave-in treatment,” says Goldstein. Although some people might experience sticker shock when they see the price of K18 (it is $75 for a small bottle, after all), it ultimately can end up being the more affordable pick because you aren’t purchasing several products for hair repair.
Now to answer the question you’re most interested in...
Is Olaplex or K18 the better hair treatment?
Soooooo, if we’re solely asking the experts, they say that K18 is hands down the better treatment. “Having used both products extensively on multiple clients with varying hair types, I always find that K18 achieves a significant—and immediate—difference in the hair structure,” says Cardenas. “I also truly became a believer when I used it on myself,” he adds. “I have curly, highlighted hair, and it always takes my curls a couple of weeks to go back to normal after a color service. The first time I ever used K18, my curls immediately came right back to life.”
Goldstein agrees. “K18 has more advanced technology, allowing for deeper repair that lasts longer.” Plus, she mentions that the quick four-minute application is really a game changer, making it easier than ever for people to restore their hair quickly.
My reviews:
K18 Biomimetic Hairscience Leave-In Molecular Repair Mask
K18 Biomimetic Hairscience Leave-In Molecular Repair Mask
Pros
- Suuuuper fast-acting (4 minutes is all you need)
- Doesn’t weigh down hair
- Makes my hair immediately softer and shinier
Cons
- A little expensive for a tiny bottle
The 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 peptide
- Size: 1.7 oz
- Usage: After shampooing, skip conditioner and run one to two pumps through your midlengths and ends, then leave in for four minutes before combing through
One 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 EDITOR
Features at a glance
| Key ingredients | K18 peptide |
| Size | 1.7 oz |
| Usage | After shampooing, skip conditioner and run one to two pumps through your mid lengths and ends, then leave in for four minutes before combing through |
Olaplex N°.3PLUS Complete Repair Treatment
Olaplex N°.3PLUS Complete Repair Treatment
Pros
- Extremely lightweight
- Great for all hair types
- More affordable for a larger bottle
Cons
- Works best in tandem with the rest of the Olaplex line
- Can be a bit time-consuming to use
This 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,.”—BG
Key features at a glance
| Key ingredients | Bis-Aminopropyl Diglycol Dimaleate |
| Size | 3.3 oz. |
| Usage | Once a week, apply to damp hair before shampooing and let sit for 10 minutes |
Final 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.
Meet the experts:
- Jessica Goldstein is a professional hairstylist and owner of J Gold Salon in Atlanta.
- Jae-Manuel Cardenas is a New York–based celebrity hairstylist.
Why 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.
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