Ragwort contains many different alkaloids, making it poisonous to animals. (EHC 80,section 9.1.4). Alkaloids which have been found in the plant confirmed by the WHO report EHC 80 are -- jacobine, jaconine, jacozine, otosenine, retrorsine, seneciphylline, senecionine, and senkirkine (pp322 Appendix II). Other alkaloids claimed to be present but from an undeclared source are acetylerucifoline, (Z)-erucifoline, (E)-erucifoline, 21-hydroxyintegerrimine, integerrimine, jacoline, riddelline, senecivernin, spartioidine, and usaramine.

Ragwort is of concern to people who keep horses and cattle. In areas of the world where ragwort is a native plant, such as Britain and continental Europe, documented cases of proven poisoning are relatively common. Horses do not normally eat fresh ragwort due to its bitter taste. It loses this taste when dried and can become a danger in hay. The result, if sufficient quantity is consumed, can be irreversible cirrhosis of the liver. Signs that a horse has been poisoned include yellow mucus membranes, depression, and lack of coordination, very much the same effect as when Mr Evans & I drink some of his parsnip wine..

Animals may also resort to the consumption of ragwort when there is shortage of food. In rare cases they can even become addicted to it. .

The danger of Ragwort is that the toxin can have a cumulative effect. The alkaloid does not actually accumulate in the liver but a breakdown product can damage DNA and progressively kills cells. About 3-7% of the body weight is sometimes claimed as deadly for horses, but an example in the scientific literature exists of a horse surviving being fed over 20% of its body weight. The effect of low doses is lessened by the destruction of the original alkaloids by the action of bacteria in the digestive tract before they reach the bloodstream. There is no known antidote or cure to poisoning, but at examples are known from the scientific literature of horses making a full recovery once consumption has been stopped.