Butterbur contains petasins, a group of bitter-tasting compounds in a class of chemicals called sesquiterpenoids. Petasine is a specific petasin considered important in butterbur. Petasins relax blood vessels and various smooth muscles in the body, such as those that are found in the uterus and lungs, according to test tube and animal studies.² Petasins are also known to reduce inflammation, as demonstrated in human studies.³ Because of these properties, butterbur might be expected to be beneficial for people with migraines and asthma. Butterbur extracts have consistently been shown to reduce symptoms in people with migraines more effectively than placebo. Butterbur has also been shown to help people with asthma, although the results have been conflicting.^{4, 5, 6} Some studies have also shown that butterbur extract works just as well as a common antihistamine drug for people with hay fever, but without causing drowsiness.

Butterbur also contains unsaturated pyrrolizidine alkaloids. These can cause severe liver damage in some people if taken for too long. Only extracts that exclude pyrrolizidine alkaloids should be used.