The root of this European-sourced herb is primarily used in medicine, although the leaf is also used.  Its Latin name is Petasites hybridus and is a shrubby plant in the Asteraceae family found throughout Europe, parts of Asia, and North America, and grows in damp, marshy areas.  It is closely related to the Tussilago genus, commonly called Coltsfoot, which is also used medicinally.  Butterbur has traditionally been used to treat headaches, ulcers, urinary tract issues, and colic.
This plant seems to act primarily as an antispasmodic and anti-inflammatory and shows some pretty impressive results as an antihistamine for allergy sufferers and as a preventive and potential treatment for migraine sufferers in clinical trials.  It is not clear exactly how this herb works for allergies, but compounds in it appear to block the action of histamine and leukotrienes, inflammatory chemicals involved in allergic reactions, reducing symptoms of allergic rhinitis.  It was shown to be as effective as pharmaceutical antihistamines but without the side effects, namely drowsiness, which is obviously desirable for daytime use.
For migraine headaches, regular use can reduce the frequency by half, and decrease the severity of the symptoms as well.  This is likely due to Butterbur’s antispasmodic and anti-inflammatory effects.
The sticking point for the use of this herb is that it contains pyrrolizidine alkaloids which can harm the liver.  Obviously, as overloaded and worked as our liver is, this is not a good thing.  Therefore, it is best to take this herb in capsule form where the alkaloids have been removed.  50 mg 3-4 times/day is the recommended dose to have effective antihistamine effects, and anywhere from 50 – 75 mg 2-3 times/day for preventing migraines.