Thanks for the research, Vicki.

The site that is selling butterbur says it's been used safely for 25 years. Another site (the link my friend had sent) gave examples of clinical trials which show butterbur helps reduce migraines.

However, one of the other sites said: "Butterbur also contains pyrrolizidine alkaloids which are associated with liver toxicity and carcinogenesis and considered unsafe."

And another site said:
"Natural petasites (butterbur) contains highly toxic alkaloids that are known to seriously damage the liver and to cause cancer. They have also been associated with potentially fatal blood clots and with chronic scarring of the lung. Even small traces of these alkaloids are considered quite dangerous."

This last paragraph sent up too many red flags.