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
UPDATE: There is a new Olaplex product coming called the Olaplex No.5P Blonde Enhancer Toning Conditioner.
Designed to complement the No.4P purple shampoo, the toning conditioner helps remove yellow tones and brassiness and freshens up blonde hair.
This is because the K18 hair treatment is meant to be used in place of a conditioner.
If you want to benefit from Olaplex’s toning range while sticking with the K18 mask, use it with the Olaplex No5.P shampoo. This is our preferred hair treatment at the moment.
And keep reading for our K18 vs. Olaplex head-to-head.
Original K18 vs Olaplex review continues below
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.
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.
Wondering how to use Olaplex No.3 for the best results? Well, 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.
No. K18 isn’t the same as Olaplex because they protect and repair hair in very different ways.
K18 and Olaplex both repair hair, but they do so in different ways. This means you could technically use both together – use the Olaplex No.3 Hair Protector before shampooing, and use the K18 Hair Treatment after.
If you’re already using Olaplex, you could replace it with K18 if you want to use just one, or can only afford to use one. However, you don’t have to.
Olaplex Number 3 has not been recalled. It’s not banned in the UK, nor does it cause infertility.
The confusion around Olaplex Number 3 being recalled is because it used to contain an ingredient that was banned in EU on 1 March 2022 called lilial.
Lilial has been shown to “interfere with reproduction processes” i.e it has been linked with infertility and it’s not longer allowed to be used in product formulations.
Olaplex removed this ingredient ahead of the ban and confirmed that all No.3 Hair Perfectors sold from January 2022 globally are lilial-free. This means the product doesn’t breach regulations and can still be sold.
You can read more in our The truth about Olaplex being banned in the UK over claims it causes infertility guide.
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