If your hair is bleached, colour-treated or heat-damaged, you've almost certainly come across three names: Olaplex, K18 and Kérastase. All three are loved by professional hairdressers and deliver genuinely impressive results — but they work in very different ways. Here's our honest comparison to help you choose the right one for your hair.
Why Damaged Hair Needs More Than Regular Conditioner
Standard conditioners work by coating the outside of the hair shaft with smoothing ingredients. This makes hair feel softer temporarily, but it doesn't address the underlying structural damage that causes breakage, dryness and dullness.
Bleaching, colouring and heat styling damage hair from the inside — breaking apart the protein bonds and molecular chains that give hair its strength and elasticity. To genuinely repair this kind of damage, you need products that work at a deeper level. That's exactly what Olaplex, K18 and Kérastase are designed to do.
At The Cosmetologists we're official authorised UK stockists of all three brands, and our professional team uses them regularly in our Hull salon. Here's what we've found.
Olaplex — The Bond Builder
Olaplex was the first brand to introduce bond-building technology to the consumer market and it remains the benchmark against which all others are measured. Its patented active ingredient works by seeking out and reconnecting broken disulphide bonds inside the hair shaft — the same bonds that are destroyed by bleach and chemical processing.
The results with Olaplex are cumulative. The more consistently you use it, the stronger and healthier your hair becomes over time. It's not a quick fix — it's a repair system that genuinely rebuilds hair structure with regular use.
Best Olaplex products for damaged hair: No.0 + No.3 used together weekly for severe damage, No.4 + No.5 for every wash, and No.8 mask fortnightly for deep moisture alongside the repair.
Best for: Heavily bleached, chemically processed or colour-treated hair that needs ongoing structural repair. Also ideal as a maintenance system to prevent future damage.
K18 — The Molecular Repair Mask
K18 takes a different scientific approach to hair repair. Rather than targeting disulphide bonds like Olaplex, K18 uses a patented peptide called the K18Peptide that works to reconnect broken keratin chains and restore the hair's polypeptide structure — the deeper molecular framework that gives hair its strength, stretch and softness.
The most remarkable thing about K18 is its speed. The K18 Molecular Repair Hair Mask delivers visible results in just 4 minutes, and many users report a dramatic transformation in texture and softness after a single use. It's leave-in, meaning you apply it after shampooing and don't rinse it out — it continues working as you style.
K18 is also completely weightless, which makes it ideal for fine or thin hair that can feel weighed down by heavier treatments.
Best K18 products for damaged hair: The K18 Molecular Repair Hair Mask is the hero product and where everyone should start. Use it after every shampoo on severely damaged hair, or weekly for maintenance.
Best for: Hair that needs fast, visible results. Particularly outstanding for fine, thin or over-processed hair. Also brilliant as a one-product solution for anyone who wants simplicity — one treatment covers repair, moisture and softness in a single step.
Kérastase — The Luxury Treatment System
Kérastase approaches damage repair differently to both Olaplex and K18. Rather than a single hero ingredient, Kérastase uses sophisticated multi-ingredient formulations developed in its Paris laboratories that combine repair with intensive nourishment, shine and manageability.
For damaged hair, the two most relevant Kérastase ranges are Résistance and Genesis. Résistance is designed for weakened, over-processed hair and uses a Fibra-Plex technology that fortifies the hair fibre and reduces breakage. Genesis targets hair that falls and breaks due to weakening at the root — ideal if you're noticing excessive shedding alongside damage.
Kérastase also offers the most luxurious sensory experience of the three — the textures, fragrances and in-use feel are exceptional, which is part of why it's been the professional salon choice for decades.
Best Kérastase products for damaged hair: Résistance Bain Extentioniste shampoo, Fondant Extentioniste conditioner and the Masque Extentioniste for weekly deep treatment. For hair fall alongside damage, switch to the Genesis range.
Best for: Hair that needs repair combined with intensive nourishment and shine. Also the best choice if you want a complete luxury haircare ritual rather than a targeted treatment system.
Olaplex vs K18 vs Kérastase — Which Should You Choose?
Choose Olaplex if your hair is heavily bleached or chemically processed and you want a proven, science-backed repair system that delivers cumulative results over time. It's the most targeted bond-repair solution available and the professional standard for chemically damaged hair.
Choose K18 if you want fast, visible results with minimum effort. The 4-minute leave-in mask is transformative and the weightless formula makes it ideal for fine or thin hair. It's also the simplest routine of the three — one product does the job of several.
Choose Kérastase if you want repair combined with luxury nourishment, exceptional shine and a complete haircare ritual. It's the right choice if your hair needs more than just structural repair — if dryness, dullness and manageability are also concerns, Kérastase addresses all of them beautifully.
Can you use them together?
Yes — and many of our customers do. A popular professional combination is using the K18 mask for rapid structural repair, then maintaining with Olaplex No.4 and No.5 in every wash and a Kérastase treatment mask weekly. There are no ingredient conflicts between the three brands.
Where to Buy Genuine Products in the UK
As official authorised UK stockists of Olaplex, K18 and Kérastase, every product at The Cosmetologists is 100% genuine, sourced directly from brand distributors. Fast UK delivery on all orders, free on orders over £50.
If you're unsure which brand or product is right for your specific hair concern, our professional team is always happy to advise. Call us on 01482 692703 or visit us at 499 Anlaby Road, Hull.
Shop Olaplex → Shop K18 → Shop Kérastase → Shop all damaged hair products →
Related Guides
Want to know exactly how to use every Olaplex product? Read our Complete Olaplex Guide for a full breakdown.
Using heat tools on damaged hair? See our ghd Straighteners Comparison to find the most hair-friendly option.
