Science is working to prove modern uses of herbs. One of these these herbs is butterbur, Petasites hybridus. Butterbur extracts contain numerous constituents, many of which have unknown effects. There are expected responses to both prescription drugs and herbal remedies. Undesired responses to drugs and herbal remedies are side effects. Pyrrolizidine alkaloids commonly occur in plants as bitter constituents to protect the plant from being eaten. Human gut bacteria reduces the toxicity of pyrrolizidine alkaloids, only to have them metabolized by the liver to toxic carcinogenic derivatives. According to scientists, large amounts of pyrrolizidine alkaloids can cause liver toxicity. Two of these pyrrolizidine alkaloids, senecionive and ingerrmine, are contained in small amounts in butterbur root and lesser amounts in butterbur leaf. This is of concern because the root is used medicinally for migraine headache prevention. Butterbur supplements identified as “pyrrolizidine free” are the best ones to use. If buttterbur is taken more than six consecutive weeks per year, a pyrrolizidine-free product must be used to avoid potential toxicity. A product free from pyrollizidine alkaloids is not necessarily free from side effects. Liver damage can result from buildup of these activated pyrollizidine alkaloids in the liver, in the form of benign and malignant tumors.