Cocamidopropyl Betaine
In plain terms
A gentle, foaming cleanser made from coconut oil. It is often added to shampoos and washes to boost lather and to soften the harshness of stronger cleansers.
What the evidence shows
Cocamidopropyl betaine is one of the milder cleansers, made from coconut oil and often added to soften harsher surfactants. The American Contact Dermatitis Society named it Allergen of the Year in 2004 after reports of itchy rashes traced to products that contained it. Later research narrowed the cause: in most cases the reaction is to two impurities left from manufacturing rather than the molecule itself, and patch-test results are hard to read because the impurities and ordinary irritation overlap. Sensitization runs from the low single digits to about seven percent of people patch-tested for it, and most people use it without a reaction. For someone with sensitive skin or a past reaction, it is one to check for on a label; for most, it is among the gentler surfactants in wide use.
On the label
It usually appears in the middle of the ingredient list in shampoos and body washes, after the water and the main cleanser.