Cocoa powder is a concentrated form of all the compounds that make chocolate a potential eczema trigger: histamine, biogenic amines from fermentation, the stimulant theobromine, and notably high levels of nickel. The nickel content is particularly relevant — nickel is one of the most common contact allergens, and eating high-nickel foods can cause systemic contact dermatitis in people who are nickel-sensitive (identifiable through a patch test). Dark chocolate and cocoa powder deliver the most nickel per serving.

True immune-mediated allergy to cocoa is extremely rare — when people react to dark chocolate, the cause is almost always histamine, nickel, or the residual milk, soy, or nut content rather than the cocoa itself. A low-nickel diet trial is worth considering for people whose eczema worsens specifically with dark chocolate consumption.