In our six-month K18 vs Olaplex review, we pit the two hair-repair champions against each other in terms of performance, value for money, and ease of use to discover which is worth your hard-earned cash
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K18 vs Olaplex: Value for money
K18 vs Olaplex: Range and availability
After years of being the cult hair-repair favourite, Olaplex is now facing stiff competition from K18; a relatively new brand that not only promises to repair hair but claims to actually reverse the damage through the use of peptides.
As you can read in our What are peptides in skincare and how do they work? guide, peptides are small chains of amino acids that, in our bodies, help produce elastin and keratin.
Keratin helps form the tissues found in our skin, but it also plays a vital role in our nails and hair.
When products containing peptides are applied to the hair, the small chains of amino acids penetrate the hair shaft and bind to the keratin proteins.
It’s these proteins that make up the bulk of the hair and help determine how healthy it is.
In this way, peptides can help repair the hair but also protect it from future damage.
K18 claims that its formula causes a long-lasting improvement to the hair, while it bemoans other hair repair products for only offering temporary fixes. In fact, it goes as far as to say that the benefits that come from other hair repair products can be “washed away”.
These are strong claims so we wanted to put the two hair repair leaders head-to-head to see which is best in our K18 vs Olaplex guide.
This is not a sponsored post or advert – we bought both hair treatments with our own money and neither Olaplex, now K18 had any input into the content.
To describe the science behind K18 vs Olaplex, we need to get a little technical about the science of hair first in order to add some context. If you don’t want, or need to read this you can jump straight to the K18 vs Olaplex head-to-head section.
The chart above showcases the main difference, in terms of price, technology, and range, and below is how we rank the two products in our K18 vs Olaplex long-term reviews.
Every cell in our body contains protein, and proteins are made up of long chains of building blocks called amino acids. In order for our bodies to repair cells and make new ones, we need to eat a diet rich in protein.
This is why protein is so important for growing children, and pregnant women. It’s why gym bros rely on it so much to help fuel the growth of their muscles during training, and it’s why protein is linked with strong hair.
Two of the most important amino acids that build the proteins in our cells are called methionine and cysteine, and these amino acids contain sulfur. Hence, sulfur is one of the most important compounds for a well-functioning body and thus lies at the very heart of healthy hair.
Each strand of hair is made up of three layers:
The health of a person’s hair largely comes from the cortex. Inside the cortex are chains of a protein called keratin that are interwoven and twisted around each other.
These chains are held together by the chemical equivalent of rungs and poles on a ladder, and this relies on something called disulfide bonds.
Disulfide bonds aren’t just found in hair. They’re found in all protein molecules and their job is to connect two sulfur atoms together. In particular, they create the bond between the sulfur-containing amino acids cysteine. When two cysteine molecules come close together, and the sulfur atoms interact, they create a strong bond between them.
“Imagine a rope. It isn’t one single fibre, but rather the interweaving of many, many much smaller strands,” explains K18. “On its own, one small string can’t support much weight; but when combined with dozens of others into a rope or cable, it can support a lot of weight. Hair is structured the same way.”
When hair is exposed to heat, bleach, the sun, or harsh chemicals, the disulfide bonds holding the keratin chains together break. This causes the two once-connected sulfur molecules to become “free.”
When this happens two potential things can happen. Either, one of the single sulfur molecules joins up with a single oxygen molecule to form a perfect pair (in this instance, no damage takes place), or a reaction takes place that creates what’s known as a sulfate group.
In this instance, the single sulfur hydrogen molecule joins up with three oxygen molecules. This results in the sulfate becoming what’s none as protonated which leads to the production of Cysteic acid. Cysteic acid then “eats” the protein out of the hair causing it to become weak and damaged.
Both K18 and Olaplex work to fix these weak and broken bonds and restore the strength and health of your hair, but they do so in slightly different ways.
Both K18 and Olaplex use patented technologies that each one claims are the best way to repair damaged hair.
We go into more detail in our Olaplex guide, but all of Olaplex’s products rely on a patented ingredient called Bis-aminopropyl diglycol dimaleate.
According to Olaplex’s patent, the two ends of Bis-aminopropyl diglycol dimaleate seek out and connect any broken bonds in the hair by forming an “artificial disulfide bridge.” This bridge allows the bonds to reform which helps strengthen the hair from within.
The stronger Olaplex treatments help grab the single sulfur hydrogen molecule to create a pair, before it bonds with the three oxygen molecules, and prevent Cysteic acid being released/from destroying the protein.
There are a couple of caveats to this. Olaplex’s patent explains that, in order to see the best results, the treatment should ideally be applied on the same day as the damage occurred. This is why hairdressers will often recommend adding an in-salon Olaplex, or other bond-building treatment, with any perming, relaxing or colouring service.
Olaplex adds that it can be applied up to two weeks after the initial damage, but it’s unclear from the patent whether this means applying it after this time will lessen the effect.
What’s more, the patent also explains that the rebuilt bonds will last “for at least one week…or two months or more” and their longevity and success may depend on the extent of the damage, when the treatment is applied, the temperature and pH level of the water used, and even the temperature of the room when the product is applied.
By comparison, K18 gets its name from the K18 peptide sequence developed by a group of researchers at the University of Minho. They engineered eight small peptides based on fragments of human hair keratin to see how well they penetrated the hair, and which formed the strongest bonds with the hair proteins.
According to the brand, the K18 peptide sequence was found to have “just the right composition of cysteines, the structure, the adjacency, and the length” to fit between the keratin chains in damaged hair. Once inside, it binds itself with the chains while helping to repair the disulfide bonds.
This peptide sequence mimics the structure of keratin in the hair and is why K18 is called a “biomimetic” hair mask. Hair can’t repair itself because it doesn’t have any live cells to make this specific keratin peptide so K18 is positioned as “the next best thing” to create a “near-natural state of strength and elasticity.”
According to one research paper, a significant feature of this peptide is that it is able to penetrate the hair in ways that existing chemicals and other treatments apparently can’t.
Another explains: “Peptides, when applied to chemically over-bleached hair and curly African hair, were shown to penetrate the hair fibre into the cortex and to bind to the hair proteins. Due to this binding, they induced a significant recovery in the strength and elasticity of severely damaged hair.”
It adds that these peptides work best at repairing disulfide bonds at neutral pH. The article also continues that the peptide method is more environmentally friendly that chemical treatments.
K18 claims this makes its hair mask more effective, and longer-lasting.
In the battle of K18 vs Olaplex, it appears that the science behind K18 is a stronger proposition. It promises to create stronger bonds, using peptides that are biomimetic and which bind to the proteins and repair the disulfide bonds. Rather than rebuilding just the bonds, as is the case with Olaplex. K18 also promises that its results are longer-lasting, although this can’t be personally verified or confirmed.
Due to their different technologies, the way you use K18 and Olaplex also have key differences.
Olaplex’s official instructions for its No.3 Hair Protector are:
The patent adds that the amount of product applied should be “sufficient to saturate hair.”
When hair is wet, it’s more ‘open,’ which means any products applied can more easily access the lower layers and make more of a difference to the cortex, where the key proteins are found. However, if it’s too wet the products can slide or wash off before they’ve had time to work their magic, so we recommend you apply Olaplex No.3 to towel-dried or damp hair. You can, in theory, use it on dry hair too but you’ll need to use more of it to achieve saturation.
You don’t need to shampoo your hair before use; simply wetting it should be enough. However, using a clarifying shampoo or similar beforehand can help remove any build-up, grease, or anything else that could impact how well the formula works.
Olaplex advises that you leave the formula on your hair for 10 minutes. The patent claims that it can be washed off within 30 minutes of application “but preferably between about five minutes and about 20 minutes. More preferably about 10 minutes after the final application of the binding agent to the hair, depending on the hair type.”
Olaplex No.3 is not a conditioner and it’s meant to be washed out of your hair so you need to follow the treatment with your usual hair-washing routine.
The official K18 mask instructions are:
When your hair is first damaged, K18 advises that you apply the mask every 4-6 shampoos consecutively. After this point, you enter the “maintenance” stage, at which point K18 recommends using the mask every 3-4 washes to protect against daily damage.
The brand suggests using a clarifying shampoo to remove any product build-up, avoiding using too much (as it can weigh down the hair and waste more product than you need), and making sure the mask sits for four minutes before you apply any other products, or attempt to style the hair. This is to give it time to work without interference.
The reason why you can’t add conditioner before applying the mask is that conditioners tend to create a barrier on the hair. This can prevent the mask from penetrating deep into the hair shaft and working as effectively as it should.
It’s quicker, requires less waiting time, and you don’t need to wash it off.
In the battle for budget, Olaplex is significantly cheaper per 30ml than the K18 mask. While this is a guide only – as neither brand sells products of the same size – 30ml of the K18 leave-in mask costs approximately £33. While 30ml of the Olaplex No.3 Hair Protector is much lower, at £8.40.
With the Olaplex No.3, you have to saturate the hair with product (as per the Olaplex patent’s guidance) meaning that use-for-use, you will likely use more. In our own tests – more on that below – we got through three bottles of No.3 using it twice a week for three weeks. That’s half a bottle per treatment, and we have thin, mid-length hair.
By comparison, we used the K18 as instructed and still have the same 50ml more than a month later.
K18 also insinuates that its results last longer than Olaplex’s. We can’t confirm this. The Olaplex patent says the results can last anywhere from a few days to months, depending on the conditions it’s used in. K18 simply calls its results “immediate, progressive, and long-lasting.” If we take this at face value, that makes K18 better value for money but it’s a grey area.
Olaplex is a lot cheaper, per ml, than K18 but K18’s mask will last longer and you use need to use less. Thus making it better value for money. K18 also claims the results are long-lasting which would add further to the value for money, but we can’t confirm this ourselves.
One area where Olaplex does dominate is in terms of the range of products that contain its bond-building technology. As a well-established brand, Olaplex sells a huge collection of different types of products to suit different needs.
We highly rate the No.9 Serum and the No.7 Bonding Oil, in addition to the No.3. We’re less enamoured of the shampoo and conditioner options; we didn’t feel they made much of a difference in comparison to the other products we’ve tested.
There are also in-salon products – No.1 and No.2 – that offer a more intense and targeted treatment but these are only available to hair professionals.
In the UK, Olaplex is sold almost everywhere from Amazon to Boots, Cult Beauty, Lookfantastic, Sephora, ASOS, and more.
By comparison, K18 is a new entrant to the hair care market so its offering is smaller.
Like Olaplex, K18 also sells a couple of professional products including the K18 hair mist and the K18 Peptide Prep PRO chelating hair complex.
In the UK, K18 is sold at Amazon, Cult Beauty, Beauty Bay, All Beauty, and more.
Olaplex offers a much wider range of products to suit different needs and budgets.
We’ve been avid Olaplex fans for years. We use the No.9 Serum daily, we use the No.7 Bonding Oil at night, and we have been using the Olaplex No.0 and Olaplex No.3 to treat our hair and repair damage religiously since we were in our 20s.
During lockdown we ravaged our hair with box dyes to the point where the front sections were springy when wet and they’d snap during brushing. For months, our hair didn’t grow and looked thin and lifeless. The only solution was to have it cut short.
For the months that followed we committed to a twice-weekly Olaplex No.3 treatment and noticed a significant difference in the health of our hair. It took around four treatments for these changes to really appear but once they did, our hair felt and looked less fragile. It looked smoother and less frizzy and it was easier to style. It also held the style better than before. However, it still didn’t grow and it was lacking in volume and movement.
That was until we started using K18. During our K18 review period, we stopped using Olaplex in order to get a more accurate view of K18’s performance. From the first use, our hair looked noticeably healthier. It not only felt smooth, but it was soft to the touch and even more manageable. Once styled it had more bounce and movement.
We only wash our hair 2-3 times a week but, as instructed, we used the K18 mask after the first six washes before switching to only using it once every three washes. And it started growing! For the first time in years, our hair grew below our chin and is still growing – albeit slowly. It also has more body than before. We’re delighted.
Based on our experience, the K18 offers more immediate and effective results but it’s clear Olaplex also works as promised. If we had to choose one, we’d choose K18. It was easier to introduce to our haircare routine, takes less time to see results and a little goes a long way. It is expensive though, and this will put it out of reach for many people sadly.
It was always going to take something significant to tempt us away from Olaplex. It’s been a favourite of ours for almost a decade and – despite the lawsuits and complaints – we’ve never experienced any irritation or problems.
Yet K18 is that something significant. In a relatively short time, it’s completely restored our hair. Our hair now has bounce, volume, and movement. It’s easier to style, and it holds the style for much longer than before. It’s also grown more in the three months we’ve been using K18 than it usually grows in six months to a year.
That said, there isn’t any reason (other than the price of course!) why you can’t use Olaplex and K18 together. They fix the hair in different ways, they’re applied differently and they offer somewhat different benefits.
You can also use Olaplex’s wider range of products with the K18 mask, if your hair doesn’t require an intense treatment or you have different needs. This could prove particularly beneficial once you leave the treatment stage of the K18 routine, and enter the maintenance stage. For instance, we’re going to carry on using our Olaplex No.9 and No.4 Dry Shampoo in conjunction with the K18 mask as they complement each other, rather than competing.
What’s more, when our next bottle of No.3 arrives, we plan to use the two treatments with one another to see if we experience any problems, or if we get the hair of our life. We’ll update this page once that test is complete.
Victoria is founder and editor-in-chief of mamabella, freelance journalist and Mum. She has a passion for empowering people to feel beautiful whatever their age, size, skin type and budget