Many types of beans contain toxins called lectins. The highest concentrations are found in kidney beans, especially red kidney beans. As few as four or five raw beans can cause severe stomach ache, vomiting and diarrhoea.
To destroy the toxins, soak the beans for at least five hours and then boil them briskly in fresh water for at least 10 minutes. Do not cook beans at a low temperature, for example in a slow cooker, as it may not destroy the toxin. Improperly cooked beans can be more toxic than raw ones. Tinned beans can be used without further cooking.