What it is
E966 is lactitol: a low-calorie sweetener and bulking agent produced by reducing lactose, which is sourced from cow's milk whey.
Why it matters
Used in sugar-free sweets, chocolates, chewing gum, baked goods, and pharmaceuticals (as a mild laxative). The dairy origin is rarely disclosed beyond the E-number itself.
What to do
Always not vegan (and not suitable for those avoiding dairy). Vegan-friendly sugar alcohols include sorbitol (E420), maltitol (E965 — usually plant-sourced from wheat or corn), and erythritol (E968 — fermented from corn).