Contra-indications of comfrey.

My research uncovered no contra-indications for comfrey if used externally. However, there are contra-indications for internal use. This is due to the pyrrolizidine alkaloids found in large part in the roots, and to a lesser degree, in the leaves of the Russian comfrey. These alkaloids have been shown to “cause liver veno-occlusive disease in humans. In this condition the small and medium veins of the liver become obstructed which results in liver dysfunction, cirrhosis and possibly death.” Pregnant women, developing fetuses and infants are particularly affected by these alkaloids. For this reason, pregnant women and infants should not use comfrey root at all, or at least for any extended length of time.

So much for the studies and the formalities. Now we will address the issue from another angle. The studies done on comfrey were conducted on 6-week old laboratory rats. These rats were fed diets high in comfrey (30-50%), and they developed tumors as a result. “Humans have never manifested tumors from regular, long-term ingestion of plants with pyrrolizidine alkaloids. Instead, humans tend to develop liver veno-occlusive disease." Since rats and humans have differing metabolisms, what is harmful to one might be harmless to the other. In addition, 6-week old rats would probably never choose to consume half of their nutrition from comfrey if they could make that decision for themselves, and humans rarely, if ever, get half of their nutrition from comfrey alone.

Three specimens of comfrey from a grower in Washington state were analyzed by an independent laboratory. One specimen had no pyrrolizidine alkaloids whatsoever, and the other two showed only minute trace amounts. It is important, however, to bear in mind that “high concentrations of many foodlike substances taken over a prolonged period will eventually reveal some minor toxic component which would not be evident with normal consumption.”

Nevertheless, plants have complex biochemistry, which often allows for small amounts of substances, which, when isolated and concentrated might otherwise be poisonous in their entirety, to be quite safe and harmless. This might account for the “lack of any conclusive cases or historical records of liver veno-occlusive disease among humans who have been known to take fairly large long-term doses of comfrey root or leaf to treat a variety of diseases.”

Additionally, recent studies have shown that not all comfrey leaf possesses pyrrolizidine alkaloids, and that which is harvested in late summer or fall seems to have only minute traces. As mentioned before, the root has higher amounts than the leaf. Dr. Christopher offers the following opinion on the pyrrolizidine alkaloids: “The root and young leaves contain a toxic alkaloid which…may cause liver damage if taken in large amounts (more than the liver can process and eliminate.) If the liver is congested or weak, it is better to use the mature leaf for internal use, avoiding the root and young leaf if possible.”