Ergotism, or "St. Anthony's Fire," is a vasospastic disorder that historically resulted from ingestion of ergot, which is produced by a rye-wheat fungus called Claviceps purpurea. Modern ergotism is most commonly the result of toxic ingestion of ergotamine alkaloids, which are used in the treatment of migraines, or toxicity from a combination of ergot-containing substances (caffeine, cigarettes, and ergotamine alkaloids). Modern ergotism is very rare (0.001-0.002% of people taking an ergot-derived medication are affected), but all clinicians should be aware of such medications' association with ergotamine toxicity.

Ergotism can take two forms: convulsive (neurologic) and gangrenous (ischemic). Convulsive ergotism shares many clinical features with serotonin syndrome, while gangrenous ergotism is a form of dry gangrene due to vasoconstriction, most often of the lower limbs. While both forms existed in epidemics historically, modern ergotism is usually related to chronic toxicity from overuse of ergotamine alkaloids and typically takes the "gangrenous" (ischemic) form. Acute ergotism, although less common, can present with signs and symptoms of both convulsive and gangrenous forms.

In both convulsive and gangrenous ergotism, the initial symptoms consist of gastrointestinal disturbances, vague flu-like symptoms, and distal extremity paresthesias. In the convulsive type, the disorder then progresses to involuntary, spastic movements, opisthotonus, hallucinations, mental status changes, seizures, and even death. Symptoms can be episodic with short periods during which the patient seems unaffected.

In the gangrenous form of modern ergotism, patients are mentally intact, and peripheral vasculature is the target of the disorder, due to alpha-adrenergic stimulation. Symptoms are typically claudication and a burning sensation in the lower limbs, decreased distal pulses, and skin that is cool to the touch with a bluish discoloration in severe cases.

Perianal ulcers and even anorectal strictures and rectovaginal fistulas have been reported in association with ergotamine tartrate suppositories, which are used for migraines. Ergotamine-induced perianal ulcers are typically painless and clear up with the cessation of the medication. Local vasoconstriction is the presumed cause.