This document is aimed at focusing on a health menace that is insufficiently recognized, in order to evaluate the health risks on the basis of published data, and to draft a set of recommendations that would help in its recognition, prevention, and control. In the present document, the alkaloids and their properties are described together with the sources of human exposure and the diseases that they produce in man and animals. The risks for human health are evaluated and recommendations are made for reducing such risks. The pyrrolizidine alkaloids with which this document is concerned are those that have previously been called "hepatotoxic" or 'nucleotoxic'. Here it is proposed to refer to them as "toxic" PAs, because of the weight of evidence now available that they produce damage in other organs as well as the liver, and the need to avoid a restrictive term. There are other types of pyrrolizidine alkaloids, such as those that occur in the plant family Orchidaceae, which are not toxic and are not discussed here.