K18 Leave-In Molecular Repair Hair Mask (50 ml)

Bleach, relaxers and hot tools break the keratin chains that give hair its strength and stretch. Conditioners and oils coat the outside so hair feels better until the next wash. K18 aims deeper. Its patented peptide, sh-oligopeptide-78, is sized and sequenced to slot into broken keratin chains and reconnect them, so the improvement is meant to last through washing rather than rinse away.

It is also unusually quick. There is no timer, cap or rinse-out step. It goes on damp hair and stays there.

Best for: blondes and anyone with highlights or a colour correction, hair that feels gummy when wet and brittle when dry, and heat-styling regulars whose ends keep snapping.

Skip it if: your hair is healthy and simply dry, in which case a good conditioner is better value. It contains wheat protein, alcohol and fragrance, so check the ingredient list if you avoid any of these.

How to use it: shampoo, do not condition, and towel-dry well. Work in one to three pumps from ends upward, wait four minutes, then style as usual without rinsing. Use for the first four to six washes, then every third or fourth wash. Buy only from the K18 storefront, because counterfeits of salon brands are common.

Pros: Patented peptide technology aimed at structural repair, not coating; Four minutes, no rinse; A little goes a long way; 50 ml lasts months on medium hair

Cons: High upfront price; Skipping conditioner feels wrong to many users; Overuse can leave fine hair stiff