Comfrey has properties that suggest it may be a unique healing agent deserving further research. Studies to determine effective dose and therapeutic action are warranted. However, constituents in comfrey represent a health risk as well. In order to minimize the risk of liver damage in people using comfrey, more research is needed to obtain an accurate assessment of toxicity. Research to support the safe use of comfrey such as how to minimize plant production of PAs and determinations of metabolism pathways in humans would be invaluable.
Although not without merit, the comfrey research presented in the scientific literature has limited value in determining safety in humans for the following reasons:
- Isolated constituents do not reflect the actions of the whole plant.
In many instances, isolated comfrey PAs were used instead of the whole plant. Research suggests, that other components in comfrey (protein, anti-oxidants) may protect against toxicity.
- Non-representative animal models used.
The predominant animal model used to test comfrey toxicity was rats. In general, rats display a different reaction to PA poisoning than humans. Further, animal species display a wide range of susceptibility to PAs and respond differently to different PAs. Given such species diversity the only way to determine human susceptibility is to test humans.
- Dosing too high.
Studies with animals used amounts of comfrey higher than the amounts typically ingested by humans.
- Route of administration wrong.
Some studies with animals injected the PAs into the animal. This route of administration would result in increased toxicity of the PAs.
- Confounding factors in poisoning reports.
Individual susceptibility to PAs can be expected, but all case reports have confounding factors (concurrent illness, use of other medications, or protein deficiency) that would increase susceptibility further.
- Extrapolation from other PA poisoning in humans not possible.
Research or case reports involving PAs other than those in comfrey can not be used since the extent of PA toxicity depends on the specific PA.
Although a safe level of comfrey consumption is probable, the use of comfrey without apparent detrimental effects doesn't mean that liver damage is not occurring. Occult damage or undesirable subclinical effects is a characteristic of many medications including the over-the-counter medications such as acetaminophen (liver damage) and ibuprofen (gastrointestinal damage). However, in the case of these legal drugs, an assessment has been made and it was decided that possible benefit outweighed possible risk. Difficulties in using comfrey safely include:
- PA levels in comfrey vary widely from plant to plant.
General guidelines for PA levels in plant parts and by growth type are available, but not a lot is known about what leads to plant production of PAs. With the evidence that one ounce of comfrey leaf can contain between 0.13 to 60 mg of PAs, use of untested plant material carries an additional risk that it may contain higher than normal levels of PAs.
- Concurrent use of certain foods or drugs may increase or reduce risk.
The use of foods or drugs that are metabolized by the same pathways may effect the toxicity of comfrey PAs. However, we have limited knowledge on PA metabolism in humans and metabolism pathways of many food constituents. We do know that in general protein deficiency (and lack of sulfur containing amino acids) increases risk. It also makes sense that concurrent use of hepatotoxic drugs would be contraindicated.
- Benefits (and risks) not clear.
Clearly defined benefits and optimal dose for these benefits have not been elucidated.
© 2004 Dorena Rode       Acknowledgment