Ingredients
Surfactants and the oral mucosa
Foam comes from surfactants, molecules whose whole job is breaking up the boundary between oil and water. The membrane around every one of your cells is exactly that kind of boundary. That coincidence is the entire subject.
What a surfactant is
One end of the molecule likes water. The other end likes oil. Neither end is happy sitting in the middle of a glass of water, so they all crowd to the surface instead. That is what makes foam, and it is also what lets them pick up greasy dirt and carry it away in water.
How much matters more than whether
Below a certain concentration, surfactant molecules drift around one at a time. Single molecules can slot into a cell membrane and unsettle it.
Above that concentration they start clustering into tiny balls, and the balls do something different: they pull fats and proteins out of the membrane entirely. Low amounts disturb a membrane. High amounts take it apart. So the amount used and how long it sits there decide the effect, not simply whether a surfactant is on the label.
Your mouth is not your skin
Skin is covered by a layer of dead, flattened cells packed with keratin. That layer is a genuine barrier and it is the reason skin tolerates detergent at all.
Most of the lining of your mouth has nothing like it, and things pass through it far more easily. That is exactly why some medicines are designed to be held under the tongue. Almost all surfactant safety data comes from skin testing, so applying it to the mouth is a bigger leap than it looks.
What has actually been measured
Experiments in the 1990s showed that sodium lauryl sulfate, the usual toothpaste surfactant, increases the shedding of surface cells inside the mouth, at the concentrations toothpaste actually uses.
Those were small studies, and they measured a change in tissue rather than an illness. What they show is that the interaction is real and detectable. What they do not show is that it causes disease.
What about the gums?
In one small human experiment, researchers placed SLS directly on the gums for 90 seconds. Gum blood flow rose for several minutes, and most participants reported a burning sensation. That is direct evidence of a short-lived irritant response.
It is not the same as showing that SLS causes gingivitis. In a larger eight-week trial of people who already had gingivitis, SLS and SLS-free toothpastes produced no meaningful difference in gum bleeding, plaque or brushing-related abrasion. The careful conclusion is that SLS can irritate gum tissue under some conditions or in some people, not that it causes gum disease.
Mouth ulcers, honestly
The most repeated claim is that going SLS-free reduces recurring mouth ulcers. Small trials have tested it and they disagree with each other. A formal review concluded there was not enough evidence to say either way.
So "limited evidence that some people may benefit" is a fair statement. "SLS causes mouth ulcers" is not. A page that quotes only the positive studies is selling you something, and that cuts both ways.
Worth knowing
- Foam is not what cleans your teeth. The bristles do that, and there is little evidence that foam improves plaque removal at all.
- If orange juice tastes wrong after brushing, that is the surfactant briefly changing how your tongue reads taste. It wears off.
- If you get recurring mouth ulcers, trying an SLS-free paste is reasonable. Just do not expect it to be settled science.
- Sensitivity varies a lot between people and there is no way to predict in advance whether you are one of the sensitive ones.
Read the full evidence review The biophysics of how surfactants meet a cell membrane, and the honest reading of the ulcer evidence.
Foam is the most recognizable thing about toothpaste and one of the least functional. It is produced by surfactants, a class of molecule whose job is to disrupt interfaces between oil and water. Cell membranes are also an interface between oil and water. This article is about that coincidence: what surfactants do to living tissue, why the lining of the mouth is more vulnerable than skin, and what the clinical evidence does and does not support.
Key points
- A surfactant is an amphiphile: a water-loving head and an oil-loving tail on the same molecule. This is what lets it lift oils into water, and also what lets it interact with cell membranes.
- Behavior changes sharply above the critical micelle concentration, the point at which free surfactant molecules begin assembling into micelles.
- The oral mucosa lacks the keratinized stratum corneum that protects skin across much of its surface, and is considerably more permeable.
- Sodium lauryl sulfate has been shown experimentally to increase desquamation, the shedding of surface epithelial cells, in the mouth.
- Direct SLS exposure can produce a brief irritant response in the gums, including increased blood flow and burning, but a larger clinical trial did not find that SLS toothpaste worsened gingivitis.
- For recurrent mouth ulcers the trial evidence is limited and mixed. A systematic review found it insufficient to draw firm conclusions, which is a weaker claim than the one usually made in marketing.
What a surfactant is
A surfactant, short for surface-active agent, is a molecule with two incompatible ends: a polar head that associates with water and a nonpolar hydrocarbon tail that does not. Because neither end can be satisfied in bulk solution, surfactant molecules migrate to interfaces, orienting with heads in the water and tails out of it. This lowers surface tension, which is what produces foam, and allows oily material to be carried away in water, which is what produces cleaning.
Surfactants are classified by the charge on the head group. Sodium lauryl sulfate, the most common in toothpaste, is anionic: its sulfate head carries a negative charge. Cocamidopropyl betaine is amphoteric, carrying both charges depending on pH. Decyl glucoside and similar sugar esters are nonionic. The charge class matters because it governs how strongly the molecule interacts with proteins, which mostly carry charge themselves.
Critical micelle concentration
Below a certain concentration, surfactant molecules exist mostly as free monomers. Above it, they self-assemble into micelles, spherical clusters with tails inward and heads facing the water. That threshold is the critical micelle concentration, or CMC, and it is the single most useful number for predicting how a surfactant will behave.
It matters biologically because free monomers, not micelles, are the species that inserts into cell membranes. As concentration rises past the CMC, the monomer concentration plateaus while micelles multiply. Micelles then do something different: they solubilize membrane lipids, extracting phospholipids and membrane proteins out of the bilayer and into the micelle interior. Low concentrations perturb membranes; high concentrations dismantle them.
This is why concentration and contact time, rather than the mere presence of a surfactant, determine biological effect, and why comparisons between products that ignore both are close to meaningless.
Why the mouth is not skin
Most surfactant safety data comes from dermatology, and transferring it to the mouth is the central error in this area. The two tissues are not equivalent.
Skin is protected by the stratum corneum, a layer of dead, flattened, keratin-filled cells embedded in a lipid matrix. It is a genuine barrier and it is the reason skin tolerates detergents at all. Much of the oral mucosa has no equivalent. The lining mucosa of the cheeks, the floor of the mouth and the underside of the tongue is non-keratinized, and its permeability is substantially higher than that of skin. This is precisely why some drugs are given sublingually: the tissue is chosen for absorption.
Two further asymmetries. The mouth is continuously bathed in saliva, which dilutes and clears an irritant faster than skin can. But it is also re-exposed on a fixed schedule, twice a day for most people, without the recovery interval that intermittent skin contact allows. The net effect is not obvious in either direction, which is an argument for measuring outcomes in the mouth rather than inferring them from skin.
The desquamation evidence
The most direct experimental work on this comes from a Norwegian group in the 1990s. Using an experimental model, they reported that sodium lauryl sulfate increased desquamation of the oral epithelium, the shedding of surface cell layers.1 A follow-up compared two toothpaste detergents in the same model and found differing desquamative effects,2 and further work examined variation in response between groups of subjects.3
These are small experimental studies rather than large clinical trials, and they measure a tissue-level effect rather than a symptom that patients report. What they establish is that the interaction is real and detectable in a human mouth at concentrations used in toothpaste. What they do not establish is that it causes disease.
More recent in vitro work has compared the irritant potency of toothpaste ingredients directly, allowing surfactants to be ranked against one another on a common measure.4 A scoping review of reported side effects of sodium lauryl sulfate in toothpaste collects the clinical observations in one place,5 and a broader literature review sets the irritation question against the functional reasons surfactants are used at all.6
The gums specifically: irritation is not gingivitis
The clearest gum-specific experiment involved 14 participants. Researchers applied 1.5% sodium lauryl sulfate directly to one side of the gingiva for 90 seconds and water to the other. SLS significantly increased gingival blood flow from approximately the second through the tenth minute after exposure. Ten participants reported burning pain, and blood flow rose in all ten. The authors interpreted the response as irritant-induced vasodilation.10 This is direct evidence of an acute gingival response, although the small study and direct application model limit how confidently it can be translated to ordinary brushing.
An older double-blind study of 98 volunteers reported an increase in the sulcus bleeding index after six weeks of using a toothpaste described as having a high concentration of surface-active substances. Microscopy also showed changes in the gingival epithelium.11 The report provides limited formulation detail, however, and does not establish that a particular surfactant caused the change.
More clinically relevant evidence is reassuring. In a double-blind randomized trial, 120 young adults with moderate gingivitis used either an SLS-containing or SLS-free toothpaste for eight weeks. Both groups improved, with no significant between-group differences in bleeding on marginal probing, plaque or gingival abrasion.12 This argues against presenting SLS as an established cause of plaque-induced gingivitis.
Irritant and contact reactions can still involve the gums. A 2026 series of 56 cases of toothpaste-induced contact stomatitis included anterior gingival lesions in 18 patients, and symptoms resolved after the suspected toothpaste was replaced.13 Every implicated toothpaste contained several shared ingredients as well as either SLS or cocamidopropyl betaine, so the study links the reaction to the complete toothpaste rather than proving that its surfactant was responsible.
The distinction matters: burning, redness, increased local blood flow and epithelial shedding are compatible with irritation. Gingivitis is usually plaque driven and is assessed through clinical outcomes such as persistent bleeding and inflammation. Evidence for the first does not automatically establish the second.
Recurrent mouth ulcers: the honest reading
The most frequently repeated claim about SLS is that avoiding it reduces recurrent aphthous stomatitis, the common recurring mouth ulcer. Several small crossover trials have tested this, with inconsistent results: some report fewer or less painful ulcers on an SLS-free dentifrice, others find no difference.
A systematic review examining the effect of sodium lauryl sulfate on recurrent aphthous stomatitis concluded that the available evidence was insufficient to support a firm conclusion.7 An independent critical appraisal summarized the position as limited evidence suggesting that some patients with recurrent ulcers may benefit from SLS-free dentifrice.8 A separate prospective controlled trial incorporating patch testing examined whether toothpastes play a role in ulcer development.9
"Limited evidence that some patients may benefit" is a defensible statement. "SLS causes mouth ulcers" is not. A page that cites only the positive crossover trials and omits the systematic review is selling something, and the same standard applies to pages arguing the opposite.
Two other reported effects deserve mention. Surfactants transiently alter taste perception, the familiar phenomenon of orange juice tasting wrong after brushing, which is attributed to disruption of lingual membranes and of the phospholipids that normally suppress bitterness. And toothpaste-associated contact stomatitis is a documented if uncommon clinical entity, discussed further in our article on preservatives and the oral mucosa.
Where this leaves it
Well supported: the biophysics. Surfactants interact with lipid membranes, that interaction is concentration dependent and changes character around the CMC, and the non-keratinized oral mucosa is more permeable than skin. Increased desquamation from sodium lauryl sulfate has been demonstrated experimentally in human mouths. Direct SLS exposure has also produced a short-lived gingival response involving increased blood flow and burning.
Not established: that surfactants in toothpaste cause clinical disease in the general population, or that SLS causes chronic plaque-induced gingivitis. The ulcer literature is small, heterogeneous and inconclusive. Individual susceptibility appears to vary considerably and is not predictable in advance.
Worth stating plainly: surfactants are in toothpaste for reasons. They help disperse the paste, lift lipophilic debris, and produce the foam most people use to judge whether brushing is working. The honest question is not which surfactant is gentlest but whether the foam is buying you anything, and there is little evidence that it improves plaque removal, which is done by the brush.
References
- Herlofson BB, Barkvoll P. Desquamative effect of sodium lauryl sulfate on oral mucosa: a preliminary study. Acta Odontologica Scandinavica. 1993;51(1):39-43. PMID 8451922.
- Herlofson BB, Barkvoll P. Oral mucosal desquamation caused by two toothpaste detergents in an experimental model. European Journal of Oral Sciences. 1996;104(1):21-26. PMID 8653493.
- Herlofson BB, Barkvoll P. Oral mucosal desquamation of pre- and post-menopausal women: a comparison of response to sodium lauryl sulphate in toothpastes. Journal of Clinical Periodontology. 1996;23(6):567-571. PMID 8811477.
- Kasi SR, Özcan M, et al. In vitro cytotoxicity (irritant potency) of toothpaste ingredients. PLoS One. 2025;20(1):e0318565. PMID 39883661.
- Kasi SR, et al. Side effects of sodium lauryl sulfate applied in toothpastes: a scoping review. American Journal of Dentistry. 2022;35(2):84-88. PMID 35506963.
- Sabri H, et al. The yin and yang of sodium lauryl sulfate use for oral and periodontal health: a literature review. Journal of Dentistry (Shiraz). 2023;24(3):262-276. PMID 37727352.
- Alli BY, Erinoso OA, Olawuyi AB. Effect of sodium lauryl sulfate on recurrent aphthous stomatitis: a systematic review. Journal of Oral Pathology and Medicine. 2019;48(5):358-364. PMID 30839136.
- Cheng LL. Limited evidence suggests that patients with recurrent aphthous stomatitis may benefit from using sodium lauryl sulfate-free dentifrices. Journal of Evidence-Based Dental Practice. 2019;19(4):101349. PMID 31843182.
- Kaya Ozden H, Selcuk AA. Is there a role of toothpastes in the development of recurrent aphthous stomatitis? A prospective controlled clinical trial with skin patch testing. Oral Surgery, Oral Medicine, Oral Pathology and Oral Radiology. 2021;131(1):43-48. PMID 33160931.
- Herlofson BB, Brodin P, Aars H. Increased human gingival blood flow induced by sodium lauryl sulfate. Journal of Clinical Periodontology. 1996;23(11):1004-1007. PMID 8951628.
- Albers HK, Jung G, Götze W. Clinical and histological studies on the effect of a surfactant-containing toothpaste on the gingiva. Deutsche Zahnärztliche Zeitschrift. 1978;33(8):559-565. PMID 278737.
- Sälzer S, Rosema NAM, Martin ECJ, et al. The effectiveness of dentifrices without and with sodium lauryl sulfate on plaque, gingivitis and gingival abrasion: a randomized clinical trial. Clinical Oral Investigations. 2016;20(3):443-450. PMID 26293981.
- Kalogirou EM, Vasilaki M, Charalampakis G, Theodosopoulou T, Tosios KI. Toothpaste-induced contact stomatitis: a retrospective study of 56 cases and scoping review. Journal of the American Dental Association. 2026. PMID 42138663.
This article is educational and describes published research on surfactants as an ingredient class. It is not medical or dental advice, not a diagnosis, and not a claim about any product. If you have recurring mouth ulcers or persistent oral irritation, see a dentist or physician, since these can have causes unrelated to toothpaste.
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