What it does
The EU's CosIng database describes CAPB as a zwitterion (an inner salt) and lists seven functions, among them cleansing, foam boosting, viscosity controlling, hair conditioning and antistatic.1
It is an amphoteric surfactant: the charge on its water-loving end depends on the pH of the product.2 In a shampoo it often plays a supporting role. Anionic cleansers such as Sodium Laureth Sulfate do most of the cleaning, and secondary surfactants, among them the betaines, are added to limit the frizz and friction those can leave on hair.2 Dermatologists writing about it attribute its wide use to the relatively mild skin irritation it causes.3
Where you find it
In shampoos, bath gels, liquid soaps, cleansers and makeup removers, and outside cosmetics in products such as toothpaste and contact lens solutions.3 Because it is not a sulfate, it can appear in shampoos sold as sulfate-free.
What studies say about allergy
Rising rates of sensitisation led to CAPB being named Allergen of the Year in 2004, and impurities left over from its manufacture are thought to play a part.3 Two come up again and again: amidoamine, a fatty acid amidopropyl dimethylamine, and DMAPA, dimethylaminopropylamine. The US Cosmetic Ingredient Review notes that both are known sensitisers, and its conclusion is that CAPB should be formulated to be non-sensitising.4
Small studies point mainly at amidoamine. Ten people who had reacted to CAPB in patch tests used a CAPB shampoo, hand soap and body wash: seven developed dermatitis, six of the nine retested reacted to amidoamine, none reacted to DMAPA, and none reacted to CAPB that was free of amidoamine.5 A London study concluded that DMAPA is unlikely to matter in CAPB of appropriate quality, and that CAPB of suitable purity is unlikely to trigger reactions even in people who appeared allergic to it.6
A large analysis questions how many reactions are allergic at all. Of 83,864 patients patch-tested with CAPB by a network of dermatology departments between 1996 and 2009, 2.16% reacted, but the results were poorly reproducible. The authors concluded that most positive reactions are presumably false positives and that true allergic reactions are very rare.7
So the evidence is contested rather than settled. It is also invisible on the pack: EU rules do not count impurities in raw materials as ingredients8, so a label can tell you that CAPB is present but not how pure it was.
How it appears on a label
As Cocamidopropyl Betaine, its name in the EU glossary.9 Related amidopropyl betaines are close relatives: the US panel assessed them together with CAPB, noting that they share the same two impurities.4
To see it in context, the SLS vs SLES guide explains the main cleansers it is often paired with, and the ingredient checker shows what every other name on a list does.