What it is
Hair is dead keratin. Once the protective outer cuticle is chipped by brushing, heat, bleach or sun, the shaft cannot heal itself. Conditioner works by leaving a thin film of positively charged ingredients (cationic surfactants such as behentrimonium, silicones such as dimethicone, and oils) on the negatively charged, damaged surface. That smooths the cuticle, reduces static, and cuts the friction of combing, which is where much breakage happens.
Rinse-out conditioners deposit a light layer; leave-ins deposit more and suit dry, curly, coily or bleached hair. Proteins, amino acids and plant oils in the formula mainly add feel and shine.
How strong is the evidencelimited
Lab studies consistently show conditioners reduce combing force and friction, and dermatology reviews describe them as the main way to limit mechanical breakage. There are few trials on outcomes people notice over months, and none showing conditioner affects hair growth or scalp health. Claims to 'repair' hair describe a temporary coating that washes out.
The claims you’ll meet4 checked
Reduces tangles, frizz and breakageHolds upCoating the shaft lowers combing friction and static; this is the core, well-understood effect.
Repairs split endsFalseSome polymers temporarily glue split ends together; they return after washing. Trimming is the only fix.
Silicones suffocate or damage hairFalseHair doesn't breathe. Heavy silicones can build up and weigh fine hair down, which a clarifying wash removes.
Makes hair grow faster or thickerUnprovenConditioner acts on the shaft, not the follicle.
What to look for4
- ✓Weight to match your hair: light rinse-out for fine or oily hair, richer conditioner or leave-in for thick, curly, coily, bleached or heat-styled hair.
- ✓Silicones (dimethicone, amodimethicone) or cationic conditioners if breakage and frizz are the problem.
- ✓A leave-in or detangler if you comb or brush wet hair, when it's most fragile.
- ✓Fragrance-free if you get rashes along the hairline, ears or neck.
What to skip3
- ×'Bond repair' or 'rebuilding' claims on ordinary conditioners.
- ×Biotin, collagen and caffeine conditioners marketed for growth.
- ×Paying extra for exotic oils; common silicones and cationic surfactants do the conditioning work.
How to use it2
- →Apply mainly to the mid-lengths and ends after shampooing; leave for a minute or two, then rinse.
- →Detangle with a wide-tooth comb while conditioner is in, starting at the ends.
Take care3
- !Rinse-off and leave-in products can cause contact allergy on the scalp edges, face and neck; stop a new product if a rash appears there.
- !Heavy leave-ins and oils on the hairline and forehead can worsen acne there; keep them to the lengths.
- !Conditioner cannot fix hair loss; see a dermatologist for thinning, shedding or patches.
The studies1 cited · checked against PubMed
Hair cosmetics: an overview ↗Gavazzoni Dias MF · International Journal of Trichology · 2015Narrative review
A dermatology review of how shampoos, conditioners, straighteners and dyes are formulated and act on the hair shaft, and their possible side effects, to help clinicians advise on hair and scalp care.