There are numerous studies now available that show the benefits of using comfrey leaf (S. officinale) as an animal food supplement. This is especially true for older plants, where the leaf is taken prior to the "10-percent" flowering stage, much like alfalfa. While the root is quite high in PAs, there are absolutely no PAs shown to exist when 3-year-old comfrey leaf is cut at this stage of growth.
All reported toxic reactions in humans (only 4!) are from individuals who took massive doses of leaf , either by juicing it, or large quantities of tablets. However, when used as a 50-percent supplement to cattle, not only are there no reported toxic reactions, apparently it is metabolized into harmless proteins, including two essential amino acids missing in alfalfa (lysine and alanine). When combined with alfalfa, this mixture becomes a "whole food" for the cow! Allantoin is also considered very important to cattle, especially those in feed lots where chance of disease and infection is so much higher.
While debate continues on the safety of comfrey, current evidence indicates that commercial comfrey preparations are not always derived from S. officinale. Uncertainties in the marketplace are compounded by errors in the scientific literature, further complicating safety evaluation. The presence of echimidine (likely the most toxic alkaloid) in commercial products has led the Canadian government to propose a general ban on comfrey. In the U.S. the American Herbal Products Association and the FDA are both reviewing the literature on comfrey to determine what action may be appropriate regarding marketing of comfrey products.