The National List of Allowed and Prohibited Substances identifies substances that may and may not be used in organic crop and livestock production. It also lists the substances that may be used in or on processed organic products. In general, synthetic substances are prohibited unless specifically allowed and non-synthetic substances are allowed unless specifically prohibited. For example, a vaccine used to prevent pinkeye in livestock is an allowed synthetic substance and arsenic is an example of a prohibited natural substance. Some substances on the National List may only be used in specific situations, e.g. only for certain crops or up to a maximum amount.