Dry & Damaged Hair · Head to head
Both target the damage bleach causes rather than masking it, but they work differently and fit different routines. Olaplex is a longer pre-shampoo treatment; K18 is a four-minute leave-in. That practical difference decides most purchases.
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Side by side
Choose Olaplex if
You wash on a schedule and can give a treatment 10+ minutes before shampooing.
Choose it if: Hair that has been bleached, permed or relaxed and now snaps mid-length
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Choose K18 if
You want the shortest possible step, or your hair is too fine for a rinse-out treatment.
Choose it if: Badly over-processed hair where you want the shortest possible treatment step
#ad · Price shown on Amazon · Affiliate disclosure for the link to Leave-In Molecular Repair Hair Mask
Both address the damage that bleaching, coloring and chemical straightening cause to the internal structure of the strand — and there is real damage to address. Studies of bleached hair using electron microscopy describe cuticle cells separating and breaking, with significant loss of the lipids that normally protect the fiber (bleaching microstructure study, PMC).
Olaplex is built around a single patented active, bis-aminopropyl diglycol dimaleate, which the company states works on broken disulfide bonds inside the hair rather than coating the surface.
K18 uses a biotech-developed peptide, which the brand states reconnects the polypeptide chains and disulfide bonds broken during bleaching, color and heat styling, and states the formula is pH-optimized to 5.0-6.0 to limit cuticle swelling while it works.
Both of those are the manufacturers' descriptions of their own patented chemistry. We have not tested either, we have no lab, and we are not going to pretend to adjudicate between two patents. What we can compare honestly is how they fit a real routine — and in practice that is what decides which one you should buy.
Olaplex No.3 is a pre-shampoo treatment. Damp hair, work it through, leave it at least ten minutes, then shampoo and condition as normal. It adds a step before your wash, which means you have to plan the wash. It also leaves hair feeling rough if you skip conditioning afterwards, because it is a treatment rather than a conditioner.
K18 is a leave-in. Shampoo, towel-dry thoroughly, one to three pumps, wait four minutes, then style. No rinse, no conditioner in between — the brand is specific that it wants clean, towel-dried hair with nothing else on it.
That constraint is the catch. If you normally go shampoo, conditioner, leave-in, oil, straight out of the shower, K18 asks you to rearrange that. If you normally wash and go, it fits perfectly.
We do not quote prices anywhere on this site — we have no verified live price data, and a number we cannot stand behind is worse than no number. What is stable enough to say is the shape: K18 is the more expensive per treatment, Olaplex the more expensive to get into if you buy the full routine, and both cost several times what a good drugstore mask costs. Check both on Amazon and decide with real numbers in front of you.
Anyone whose hair has never been bleached, permanently colored or chemically straightened. Bond chemistry addresses bonds broken by chemical processing. If your hair is rough and dull because it is dry, you are buying a solution to a problem you do not have, and a moisture mask at a quarter of the price will make a bigger visible difference.
Also worth knowing: neither is a substitute for the wash step. If you are using a sulfate shampoo on bleached hair, fixing that is cheaper and does more than either of these — see shampoos for damaged hair.
We have not used these products ourselves. They are chosen from what is in the formula, what those ingredients are documented to do, and whether that matches the problem this page is about — and every product below links to the manufacturer's own page so you can check the formulation claims yourself. The full method is here.
In detail
You wash on a schedule and can give a treatment 10+ minutes before shampooing.
Who it suits: Hair that has been bleached, permed or relaxed and now snaps mid-length
Skip it if your hair has never been colored, bleached or chemically straightened — your problem is moisture, and a mask will do more for less.
#ad · Price shown on Amazon · Affiliate disclosure for the link to Nº.3 Hair Perfector
You want the shortest possible step, or your hair is too fine for a rinse-out treatment.
Who it suits: Badly over-processed hair where you want the shortest possible treatment step
Skip it if you layer several products straight out of the shower — this one needs bare, towel-dried hair to work as directed, and working around that is more trouble than a rinse-out mask.
#ad · Price shown on Amazon · Affiliate disclosure for the link to Leave-In Molecular Repair Hair Mask
How we picked
Most hair care roundups are written in a "we tested these" voice. We have not, and saying so is the point. These picks are chosen the way an informed shopper would choose them: read what is actually in the formula, check what those ingredients are documented to do, match that against the specific problem the page is about, and confirm the product is still being sold.
Each pick gets a role — the drugstore option, the one for high porosity, the gentlest of the set — rather than a score out of ten, because a score would imply a measurement nobody took. Nobody pays for inclusion or placement, and when the cheap product is the right answer we say that too. The full method is on one page.
Questions
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Receipts
Claims about what an ingredient does are cited to dermatology bodies or published research. Claims about what a product contains are cited to the manufacturer's own page and labeled as the manufacturer's claim. Where the evidence is thin, we say so on the page rather than rounding it up. Read the full method.