Cocamidopropyl betaine

What it is

Cocamidopropyl betaine (CAPB) is a zwitterionic (amphoteric) surfactant made from coconut fatty acids reacted with 3-dimethylaminopropylamine (DMAPA) to form an amidoamine, which is then converted to the betaine. It is the most frequently used amphoteric in shampoos, typically as the co-surfactant at about 1-5% alongside an anionic primary (Thompson 2023). It is also used in body washes, baby products, toothpaste and contact-lens solutions (Jacob 2008). CAPB appeared in 17.6% of 1,470 "clean" beauty products sampled in 2022 (Tran 2022) and was the second most common known allergen in a US drugstore shampoo database (Zirwas 2009).

How it works

• Mildness: CAPB forms mixed micelles with anionics and lowers their charge density, reducing irritation (Thompson 2023). In human skin in vitro, raising the fraction of lauramidopropyl betaine (a close relative) reduced SDS penetration steadily (Morris 2021).
• Thickening and foam: mixed micelles grow into worm-like micelles that build viscosity, and CAPB's dual charge lets molecules pack tightly at the air-water interface, boosting foam (Thompson 2023).
• Sensitization: the allergens implicated are mainly the process impurities amidoamine (cocamidopropyl dimethylamine) and DMAPA (Burnett 2012; Fowler 1997; Iijima 2023).

What the studies show

• A CIR safety assessment found CAPB and related amidopropyl betaines have no significant toxicity other than sensitization from DMAPA and amidoamine impurities, and are safe when formulated to be non-sensitizing (Burnett 2012).
• In patch-test network data from 83,864 patients, 2.16% were positive to 1% CAPB, but reproducibility was poor (kappa 0.29 on simultaneous testing and 0.12 on retest). The authors concluded most positives are false and true allergy is very rare. Risk was higher in men, people 40 or over, atopic dermatitis, scalp dermatitis and hairdressers (Schnuch 2011).
• Among 975 consecutive patch-test patients in North America, 15 were positive to CAPB, 25 to amidoamine and 18 to both, with the head and neck the most common site (Fowler 2004).
• In a use test of 10 patients, 7 of 10 CAPB-positive patients reacted to CAPB products in use, 6 of 9 reacted to amidoamine, and none reacted to amidoamine-free CAPB (Fowler 1997).
• In 30 Japanese patients with scalp dermatitis, 78% tested positive to their own detergents and 50% reacted to DMAPA, to a lauramidopropyl dimethylamine impurity, or to both (Iijima 2023).
• A review estimated sensitization prevalence at 3.0-7.2% and noted CAPB was named the American Contact Dermatitis Society's Allergen of the Year in 2004 (Jacob 2008).
• In a double-blind randomized patch-test study of 47 surfactant-allergic patients, co-reactivity was high among CAPB, amidoamine, DMAPA and oleamidopropyl dimethylamine (Grey 2016).
• Among 43,478 German patients, "allergic" CAPB reactions correlated with reactions to the irritant SLS, consistent with irritant false positives (Forkel 2026).
• A 2024 toxicological assessment found mild skin and eye irritation and skin sensitization, with a margin of safety above 100 at up to 6% leave-on and 30% rinse-off (Kim 2024).
• In 105 patients patch-tested with eight cleansers, CAPB from the Italian standard series gave 5 positive reactions, while none of the 8 newer surfactants did (Corazza 2010).

How it's used in studies

CAPB is a co-surfactant at about 1-5% in shampoos (Thompson 2023), and a 2024 assessment considered rinse-off use up to 30% (Kim 2024). Patch testing uses 1% CAPB in water (Schnuch 2011) plus amidoamine and DMAPA separately.

Side effects and safety

• Allergic contact dermatitis is rare but well documented, typically on the scalp, face, eyelids and neck. Prevalence estimates range from about 2% of patch-tested patients (with many false positives) to 3-7% in older reviews (Schnuch 2011; Jacob 2008). Most true reactions are to the impurities amidoamine and DMAPA, and higher-purity grades reduce risk (Fowler 1997; Burnett 2012).
• Cross-reactions: people allergic to CAPB often react to other amidoamine-type ingredients, including oleamidopropyl dimethylamine and some conditioner ingredients (Grey 2016).
• Irritation is mild. Amphoterics are why "no tears" baby shampoos sting less (D'Souza 2015).
• A 2023 review states commercial CAPB is believed to contain under 0.002% DMAPA and under 0.3% amidoamine (Thompson 2023).
• It is a permitted cosmetic ingredient in the US, EU and UK.

Bottom line

CAPB is a genuinely useful mildness booster: a sulfate shampoo with a good dose of CAPB is gentler than one without. For most people it is not a concern. If you have a persistently itchy or eczematous scalp, hairline, eyelids or neck that does not respond to dandruff treatment, CAPB-type allergy is worth testing for, and an allergic person must avoid CAPB and related amidoamine ingredients in both shampoo and conditioner. How common true allergy is remains debated because patch-test reproducibility is poor.

Fun facts

• CAPB was the American Contact Dermatitis Society's 2004 Allergen of the Year, yet a German network analysis of almost 84,000 patients concluded most positive patch tests to it are probably false positives (Jacob 2008; Schnuch 2011).