Ergot poisoning has been proposed to explain werewolf episodes in Europe in the 18th and 19th centuries as both a cause of an individual believing that he or she is a werewolf and of a whole town believing that they had seen a werewolf.

During the period from 1520 to 1630 there were over 30,000 werewolf trials in France alone. Most of the people who were tried as being werewolves were poor, and came from lowlands with elevations less than 500 feet above sea level. Rye bread was a staple for the poorer people of France, and after cold winters the rye developed the Ergot fungus, a widespread parasite of cereal grains.This recent theory explains that people would hallucinate due to eating bread made from ergot infected rye, and therefore behave like werewolves as being.

Among the symptoms of this virulent, and often lethal, condition are: disruption of motor control functions, causing tremors and writhing, wry neck, convulsions, rolling eyes, and speechlessness; dizziness, confusion, hallucinations, panic attacks, and delusions; extreme thirst, uncontrollable appetite; feelings of extreme heat, or even cold, with itching and tingling, swelling and blistering of the skin.

There was a significant outbreak of ergot-poisoning in France in the early 1950's. Symptoms of ergot poisoning include hallucinations. The 1950's French victims reported 'being chased or attacked by horrible beasts', 'terror of the dark', and 'feeling that my body was not mine' together with tingling/burning sensations in the extremities and the scalp. Another man with ergot poisoning reportedly escaped from seven straight jackets, lost all his teeth biting through a leather strap, and bent two iron bars on the hospital window while trying to flee from a tiger he believed to be chasing him.

Rye grass is by far the most widespread species parasitized, though wheat and barley are also commonly affected. The 'ergots' appear as a blackish-purple club, shaped growth on the tops of the rye where the seeds are, and are referred to as "heads of ergot".  From these heads sprout the Claviceps purpurea fungal fruiting bodies which has long stems with bulbous heads when seen under a strong glass or microscope. The growth of ergot is stimulated by certain atmospheric conditions: it grows best in overcast and damp weather. Epidemics have been linked to volcanic eruptions, particularly in Scandinavia; and the presence of nearby marshland or lakes is enough to moisten the air sufficiently to facilitate the growth of ergot.

Ergot naturally produces a wide range of chemical compounds, the ones of relevance here are collectively known as the "Ergot Alkaloids", and include ergotamine, ergosine and beta-ergosine, ergonine, ergovaline, ergostine, ergotine and beta-ergotine, ergocornine, ergocristine, ergocryptine and beta-ergocryptine. These compounds all have some degree of psychoactivity.  Lysergic Acid Diethylmide (LSD) was first synthesized from ergot compounds. The psychoactive components of ergot are not broken down by heat, so it is fair to assume that they would be present in bread baked from flour milled and ergotized grains.

But Ergot Fungi is essentially poisonous before being hallucinogenic and the transformation into LSD is a complex chemistry procedure that cannot occurs ® naturally ¯.
 
Moreover, legends of animal transformations, as well as hysteria and superstition in general, have existed across the world for all of recorded history. Even if ergot poisoning is found to be an accurate explanation in some cases, it cannot be applied to all instances.