Sodium Lauryl Sulphate (SLS): What It Does and Who Should Watch It
Sodium lauryl sulphate (SLS) is the foaming agent in many cleansers. A plain-English UK guide to what it does, how it differs from SLES, and why sensitive skin sometimes reacts.
What it is
Sodium lauryl sulphate, almost always shortened to SLS, is a surfactant. Surfactants are the ingredients that make a product foam and that lift away oil, dirt and product build-up so they can be rinsed off. SLS is one of the most effective and inexpensive surfactants available, which is why it became a standard ingredient in cleansers of all kinds.
It is a strong detergent. That strength is a feature when the job is to cut through grease and generate a satisfying lather, and it is the source of the concern when it comes to sensitive skin, because a powerful cleanser can remove more than just the dirt.
Why it is used in skincare and household products
SLS is used because it does its job well and cheaply. It produces the rich foam many people associate with a product working, it cleans thoroughly, and it is stable and easy to formulate with. That is why it turns up in shampoos, foaming face and body washes, bubble bath, and toothpaste, and in many household cleaning products too.
The foam itself is largely cosmetic; a product does not need to lather heavily to clean. But because shoppers associate lather with cleanliness, foaming surfactants like SLS have remained popular even where a gentler, lower-foaming option would clean just as well.
The UK and EU rules
Sodium lauryl sulphate is a permitted cosmetic ingredient in the UK and EU. It is not one of the 26 named fragrance allergens, it is not banned, and it is not subject to the leave-on restriction that applies to certain preservatives. Its role and its irritancy are well understood, and it is regarded as safe to use in the way it normally is, which is predominantly in rinse-off products where contact with the skin is brief.
Where SLS is used it must be listed by its INCI name in the ingredients, so anyone wishing to avoid it can find it. Its regulatory status reflects the fact that its main drawback is irritation of susceptible skin rather than a population-wide safety hazard.
Who reacts, and why
The key distinction with SLS is that it is an irritant, not a classic allergen. It does not usually work by sensitising the immune system the way fragrance or methylisothiazolinone do. Instead, it can directly disrupt the skin’s protective barrier and strip away natural oils, which on vulnerable skin leads to dryness, tightness, redness and irritation.
This matters most for people with eczema, very dry skin or a sensitive scalp, and for uses where SLS stays in contact with the skin longer or is used at higher strength. In fact, SLS is used in dermatology research as a standard irritant to test skin barrier function, precisely because its irritant effect is so reliable. For people with a resilient skin barrier, brief rinse-off contact with SLS in a shampoo or wash is generally not a problem. Because it is an irritant rather than an allergen, the reaction depends heavily on dose, concentration and contact time.
How to spot it on a label
Look for Sodium Lauryl Sulphate, the short form SLS, or the chemical name Sodium Dodecyl Sulfate. Note the spelling can appear as sulphate or sulfate depending on the product. Very often you will instead see the milder relative Sodium Laureth Sulphate (SLES), which is a different, gentler ingredient.
SLS hides in foaming cleansers, shampoos, body washes, bubble bath and toothpaste. If a product foams generously, a strong surfactant of some kind is doing the work, so it is worth reading the list to see which one.
A balanced, honest take
SLS attracts a lot of dramatic claims online, some of which go well beyond the evidence. It is not a poison, it does not cause serious disease at the levels used in cosmetics, and much of the strongest language about it is unwarranted. The accurate, honest description is narrower and less exciting: SLS is an effective cleanser that is also a known skin irritant, and whether that matters depends entirely on your skin and how the product is used.
For someone with tough, unproblematic skin, an SLS shampoo is unlikely to cause any trouble. For someone with eczema, a dry sensitive scalp or an easily upset barrier, SLS in a daily cleanser can genuinely contribute to dryness and irritation, and switching to a milder surfactant often helps. Both of those things are true, and the sensible reading of SLS lands in between the scare stories and the dismissals.
What to do if you’re avoiding it
If SLS seems to dry out or irritate your skin, the practical response is simple. Look for products that use gentler surfactants: sodium laureth sulphate (SLES) is a milder relative, and there are various sulphate-free options built around softer cleansing agents. Treat sulphate-free as a starting point rather than a guarantee, and judge products by how your skin actually feels after use. Reducing how often you use strongly foaming cleansers, and following washing with a good moisturiser, also helps a stripped barrier recover.
If you have persistent dryness, an irritated scalp or eczema that is not settling, see your GP, pharmacist or a dermatologist for tailored advice. This is a guide to understanding and reading labels, not medical advice.
Check products against your list
To check a product, a free browser extension like Clearya flags ingredients of concern automatically as you shop online, and INCIBeauty lets you look up a product and read plain-language notes on each ingredient, with a community that rates them. These rate a product on general criteria rather than against your own list.
Once you know what you are screening for, a personal-list app like ClearaScan lets you save sodium lauryl sulphate once and scan any product to flag only your triggers. It also keeps a Reaction Journal for flare-ups, a shared Care Circle so family or carers can scan for you, and a Trusted Products list for items you have cleared, and it is currently in early access. (Disclosure: our editor co-founded ClearaScan, and we are not paid to mention the others.)
The Sensitive Skin Ingredient Decoder
A free one-page guide to reading a skincare label fast: the irritants to watch, the fragrance loopholes, and what "fragrance-free" and "hypoallergenic" really mean.
Common questions
Is sodium lauryl sulphate bad for your skin?
For most people with resilient skin, SLS in a rinse-off product is not a problem. It is a strong cleanser, so on already-dry, eczema-prone or sensitive skin it can strip natural oils and cause dryness, tightness or irritation. It is an irritant rather than a classic allergen, and the effect depends on how much is used and how long it stays on the skin.
What is the difference between SLS and SLES?
Sodium laureth sulphate (SLES) is a gentler, ethoxylated relative of SLS that tends to be less stripping while still foaming well. Many sensitive-skin ranges use SLES or other milder surfactants in place of SLS. Both clean effectively, but SLES is generally kinder to the skin barrier.
Does SLS cause mouth ulcers in toothpaste?
Some people find that SLS-free toothpaste reduces mouth ulcers, and there is limited evidence pointing that way, but it is not settled. If you get recurrent ulcers, trying an SLS-free toothpaste is a low-risk experiment, and a GP or dentist can advise if the problem persists.
Is SLS an allergen I can be patch tested for?
SLS is primarily an irritant, not a true allergen, so it behaves differently from ingredients like fragrance or preservatives. It is actually used in research as a standard irritant. Patch-test clinics sometimes include it as an irritant control rather than to diagnose allergy. If you keep reacting to washes, a GP or dermatologist can help work out why.
Is SLS-free or sulphate-free skincare better for sensitive skin?
It can be, for people with dry, eczema-prone or sensitive skin, because milder surfactants are less likely to strip the skin barrier. But sulphate-free is a marketing term, and the replacement surfactants vary, so read the whole ingredients list and judge by how your skin actually responds rather than by the front-of-pack claim.