Sodium Lauryl Sulfate.

What it does

It is so reliably irritating to skin that dermatology research uses it as a standard reference irritant to benchmark other substances in patch tests. It is a cleansing agent; its irritancy depends heavily on how it is formulated.

Benefits

• Powerful cleanser that removes oil and grime efficiently
• Generates abundant, creamy, luxurious foam
• Inexpensive and widely available, useful across cleansers and toothpastes
• Can be made considerably milder when blended with gentler co-surfactants

Side effects

• A primary irritant, so well established at irritating skin that it is the standard control in dermatological irritation studies
• Drying and defatting; removes natural oils and can worsen problems for those with sensitive or hypersensitive skin
• Prolonged contact increases the chance of redness and irritation; not ideal for leave-on or long-contact use

Who it's for

Resilient, oily skin and rinse-off products where strong foaming and deep cleansing are wanted, especially when SLS is buffered by milder surfactants