If you've ever had a dermatologist test how irritating a new skincare ingredient is, there's a good chance the comparison substance on your other forearm was sodium lauryl sulfate. It's not there by accident — SLS irritates skin so consistently and dose-dependently that it's become the standard yardstick researchers use to measure everything else against.
What SLS actually is
Sodium lauryl sulfate (SLS) and its close relative sodium laureth sulfate (SLES) are anionic surfactants — detergents that lower the surface tension of water so it can lift oil and grime off a surface, and foam while doing it. That foaming action is why SLS ended up in toothpaste, shampoo, body wash and dish soap: it feels like cleaning, because suds have become culturally shorthand for "working." Foam itself doesn't clean anything on its own; it's mostly a sensory cue.
The mechanism of irritation
SLS doesn't just remove dirt and oil — it also strips away some of the skin's own protective lipid barrier and denatures surface proteins on contact.1 In the mouth specifically, this same detergent action has been linked to irritation of the oral mucosa, and to worsening the frequency and severity of recurrent aphthous ulcers (common mouth ulcers) in people already prone to them.
This is also precisely why the Repeat Insult Patch Test and similar standardised irritation assays use SLS at defined concentrations as the reference irritant: researchers need a substance with predictable, repeatable, dose-responsive irritation to calibrate how everything else compares. SLS was chosen for that role specifically because it reliably irritates skin — it isn't a borderline or disputed effect.
Why "rinse it off" doesn't fully solve it
Toothpaste and shampoo are rinse-off products, which does reduce exposure time compared to something left on the skin. But the oral mucosa and the thin skin around the mouth are more permeable and more sensitive than most body skin, and contact happens daily, often twice a day, for years. For most people with resilient skin and no history of mouth ulcers, that's a manageable trade-off. For people with sensitive gums, eczema-prone skin, or a tendency toward recurrent mouth ulcers, it's a more direct and avoidable irritant.
What we use instead
Our tooth powder doesn't foam, because it doesn't contain SLS or any other detergent — the cleaning is done mechanically, by the kaolin clay and sodium bicarbonate, not chemically by surfactant action. Our shower soap uses traditional saponified oils (olive, coconut and shea butter) to produce a gentler lather without SLS. Neither relies on stripping the skin's own barrier to feel like it's working.
The honest trade-off
SLS-free products generally foam less, and for anyone used to a thick lather as the signal that a product is "working," that takes some adjustment. We think that's a reasonable trade for not routinely stripping the protective barrier on skin and gums you use every single day — but it is a trade-off, not a free upgrade, and we'd rather say that plainly than oversell it.