What it does
The EU's CosIng database describes it as the sodium salt of lauryl sulfate and lists four functions: cleansing, foaming, cleansing surfactant and denaturant.1 It is an anionic surfactant, one of the negatively charged group that does most of the cleaning in shampoos.2
Anionic cleansers are very good at removing sebum and dirt, but they can leave more negative charge on the hair surface, which adds frizz and friction. Formulators often add milder secondary surfactants, such as amphoteric ones, to limit that.2
Where you find it
In haircare, look for it in shampoos. A 2025 review of hair care notes that clarifying shampoos containing sulfates such as SLS are often used to remove silicone build-up, and warns that using them too often may dry the hair.3
Its relatives share the job. The US Cosmetic Ingredient Review assessed SLS together with Ammonium Lauryl Sulfate, then relied on those data to assess a group of related alkyl sulfates, including Sodium Coco-Sulfate, which it found in use at 0.1% to 29%, mainly in soaps and shampoos.4
What the studies say about irritation
SLS is the standard irritant in skin irritation research.5 The European Society of Contact Dermatitis has published guidelines for SLS exposure tests in people, so that studies which provoke irritation on purpose follow a uniform method.6
In a patch-test study comparing it with SLES and an alkyl polyglucoside at 0.125% to 2%, SLS produced a pronounced reaction, SLES a far milder one, and the glucoside one that was hard to detect. Skin exposed to SLS was still losing extra water on day 10 at every concentration tested.5
Those patches stayed on the skin for 6 to 24 hours, and the study found exposure time and concentration matter most.5 A shampoo is rinsed off within minutes and its label gives no concentration, so the results compare ingredients, not products.
SLS does not appear in the EU cosmetics regulation's annexes of banned or restricted substances.7
How it appears on a label
As Sodium Lauryl Sulfate, its name in the EU glossary.8 Do not confuse it with Sodium Laureth Sulfate: the "eth" marks a lauryl alcohol that was ethoxylated before sulfation, a different ingredient.9 The SLS vs SLES guide compares the two.
A sulfate-free shampoo is one made without sulfate cleansers such as SLS.2 The claim does not say which cleanser replaced it; what sulfate-free means lists the usual alternatives.