http://www.bccancer.bc.ca/PPI/UnconventionalTherapies/Comfrey.htm

This link reviews the history of comfrey use, the useful components (that could assist in bone healing), the liver toxicity (which is sometimes related to low doses), the carcinogenic properties, danger in pregnancy, and to breastfed infants, the variance of toxicity in plant species and plant parts, and the contamination of herb supples with other plant material (such as foxglove).

I'm all for using simple remedies where they are safe and effective, but it is important to really know what one is ingesting. I keep in mind that our modern lifespan far outdoes that of the average mediaeval herb user, evilbigpharma notwithstanding.