Recently, reports of the toxic effects of pyrrolizidine alkaloids in comfrey have led some herbalists to be wary of using it internally. (wellnessmama.com)
For example, in Germany, there is a safety limit to the level of pyrrolizidine alkaloids allowed in butterbur products. (verywellhealth.com)
Some butterbur preparations do contain chemicals called pyrrolizidine alkaloids (PAs), which can damage the liver and cause other serious harm. (mindbodygreen.com)