Sodium Lauryl Sulfate (SLS) in Toothpaste – Why Is It Used, and Is It Harmful? | The Toothpaste Trilogy, Part III
Have you ever wondered why toothpaste foams and tingles so intensely in your mouth while you brush? Sodium lauryl sulfate is largely responsible for this phenomenon—an ingredient still found in most commercial toothpastes, but one that is also the subject of lively debate. Why was it first used, and why is it still around today?
What is sodium lauryl sulfate (SLS)?
Sodium lauryl sulfate (Eng. sodium lauryl sulfate, SLS) is a so-called surfactant. It reduces the surface tension of liquids, making it an excellent cleansing and foaming ingredient.
SLS is a very common ingredient in the cosmetics and hygiene industry: in addition to toothpaste, it is used in products such as shampoos, shower gels, and facial cleansers to create a rich lather and remove oil and dirt.
Why was SLS originally added to toothpaste?
The history of toothpaste dates back thousands of years, but its modern form began to take shape at the turn of the 19th and 20th centuries.
Originally, ordinary soap was used in toothpastes to create lather and provide cleaning power. However, the soap often made the paste taste bad and reacted easily with other ingredients. The alkalinity of soap can also adversely affect the pH balance of the mouth. In the 1930s and 1940s, the chemical industry developed synthetic surfactants, among which SLS proved to be a more effective and neutral-tasting alternative to soap.
A Psychological Trick from 100 Years Ago: Where Does “Freshness” Really Come From?
Consumer psychology also played a role in SLS’s breakthrough. In his book *The Power of Habit*, journalist and author Charles Duhigg describes how, in the early 20th century, brushing teeth became a routine for the entire population by creating a tangible reward. Although the benefits of brushing teeth were clear, it proved surprisingly difficult to get people to brush their teeth regularly. One challenge was that there was no immediate reward for brushing. According to the theory presented by Duhigg in his book, for a new behavior to take root, it requires both a trigger to initiate the action and an immediate reward following the action. Together, these create a subconscious “craving” to perform the action in question.
The key factor in the success of Pepsodent toothpaste turned out to be the small amounts of peppermint oil, citric acid, and other ingredients that had originally been added for flavor and texture. However, what proved even more significant was the refreshing sensation they produced after brushing. Mint and menthol activate the TRPM8 cold receptors in the mouth. This tricks the brain into believing that the temperature in the mouth has dropped, creating a cool and refreshing sensation. The foaming caused by SLS further reinforced the impression of cleanliness, and as a mild irritant, SLS also causes a tingling sensation.
When the cooling sensation of mint and the micro-tingling caused by SLS combine with a rich lather, that familiar “aggressively fresh” sensation is created. This sensation served as a reward for brushing our teeth and taught our brains that our mouths were now clean—even though the sensation actually had nothing to do with actual cleanliness or the effectiveness of the toothpaste. Furthermore, if that sensation was missing in the evening, you’d realize you’d forgotten to brush your teeth and still had time to correct your mistake.
Why is SLS still used?
Today, SLS is still used in many toothpastes for practical reasons:
- Distribution in the mouth: The foam helps the toothpaste spread evenly throughout the oral cavity—including between the teeth and along the gumline—delivering important active ingredients, such as fluoride.
- Removes plaque and food debris: As a surfactant, SLS breaks down and removes dental plaque and fat-soluble deposits in the mouth, making them easy to rinse away.
- Affordable and effective: SLS is a very affordable, effective, and stable raw material
Did you know? SLS temporarily blocks the “sweet” taste receptors on the tongue and breaks down the phospholipids that normally mask a bitter taste. This is why orange juice or other tart drinks may taste bad right after brushing your teeth.
Instead of SLS, some products may also use other surfactants. For example, sodium olefin sulfonate (Sodium C14-16 Olefin Sulfonate) has very similar properties. It may be slightly more biodegradable and environmentally friendly, but often the reason for using it is so that the product can be marketed as “sulfate-free,” even though the difference is largely technical.
SLS isn't right for everyone – When should you choose a non-foaming toothpaste?
Although SLS is safe for most people, its mucous membrane-dissolving and drying effects may be too strong for those with a more sensitive mouth. It is recommended to avoid SLS, especially in the following situations:
- Canker sores (mouth ulcers): Studies have shown that SLS can damage the protective layer of the oral mucosa in sensitive individuals, thereby increasing the risk of recurrent, painful canker sores.
- Dry mouth (xerostomia): Because SLS is a drying agent, it may worsen the symptoms of dry mouth.
- Sensitive or sore mucous membranes: If the mucous membranes in your mouth peel easily, sting, or become irritated when you brush your teeth, the problem may be the foaming agent in the toothpaste.
- For Children: Many children's toothpastes contain no SLS or other foaming agents at all.
If you decide to switch to an SLS-free (non-foaming or mildly foaming) toothpaste, you might be in for a surprise: the cleaning results may feel strange or even ineffective at first, since your brain has gotten used to expecting foam and a tingling sensation. However, this is merely a matter of psychological habit.
The cleaning power and the protection provided by fluoride are roughly the same even without SLS—without any unnecessary irritation to the mucous membranes. After a few weeks of getting used to it, your mouth feels just as clean, but often much more comfortable and moisturized.
