Cocamidopropyl Hydroxysultaine
In plain terms
A gentle coconut-derived cleansing agent (an amphoteric surfactant) used to soften harsher detergents and boost foam in shampoos and washes. Its safety turns on manufacturing cleanliness, since the same leftover amines that make its cousin cocamidopropyl betaine a known allergen can appear in it too.
What the evidence shows
Cocamidopropyl hydroxysultaine is an amphoteric surfactant made from coconut oil, used as a secondary cleanser and foam booster that makes a formula feel milder. On its own it is gentle: the US cosmetic-ingredient panel found it was not a skin sensitizer in animal or human testing, with only slight irritation after repeated patches, and it did not irritate skin even at high strength in animal tests. The reason it still lands at depends is the same one that dogs its better-known cousin, cocamidopropyl betaine. Both are made through a process that can leave behind two trace amines, amidoamine and dimethylaminopropylamine, and it is those leftover amines, not the surfactant itself, that drive most cases of allergic reaction. Contact-allergy clinics find that people who react to these coconut cleansers are usually reacting to the residual amine. Good manufacturers keep those impurities very low, with suppliers reporting the key one at a few parts per million, and a clean batch is a mild, well-tolerated cleanser. So whether it suits you depends on the purity of the batch and whether your skin already reacts to this family of cleansers.
On the label
Listed as cocamidopropyl hydroxysultaine, usually in the middle of a shampoo or body-wash ingredient list among the milder cleansers. If you have reacted to cocamidopropyl betaine or to coconut cleansers before, this one can cross-react, so patch test; the reaction is to trace manufacturing amines, so a well-made product is the gentler option.