The fungus produces substances (alkaloids) termed ergotamines that constrict blood vessels and cause the muscle of the uterus to contract. The ergotamines have been much used (and been very useful) for the treatment of migraine. They have also been used (and misused) as abortifacients (agents of abortion).
In excess, ergotamines are highly toxic and cause symptoms such as hallucinations, severe gastrointestinal upset, and a type of dry gangrene. Chronic ergot poisoning (ergotism) was rife during the Middle Ages due to the consumption of contaminated rye.
St Anthony is one of a number of saints whose names have entered the annals of medicine. Others include St. Guy, St. John and St. Vitus. St. Anthony's fire referred not only to ergot poisoning but also to erysipelas, a type of spreading hot, bright red strep skin infection.
For more about this fascinating disease, see our article on St. Anthony's Fire -- Ergotism .