PAs in extremely large doses or over long periods of time may cause potentially fatal damage to the liver.
Many leading herbalists and traditional healers question the warnings, pointing to laboratory tests that show only minute levels of PAs in random samples of comfrey preparations.
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Still, many herbalists recommend that comfrey preparations should not be taken internally because of the possibility of liver disease and damage.
Comfrey should also not be used by pregnant or nursing women.
In another study with over 300 participants showed that comfrey leaf treatments of varying types (ointments, salves, compresses and other topical applications), were very effective in treating eczema, dermatitis, viral skin infections and ulcers of the lower leg.
More recent research in the United States has shown that allantoin, one of comfreys main constituents, breaks down red blood cells, which could account for its ability to help heal bruises and contusions.
Recently, reports of the toxic effects of pyrrolizidine alkaloids in comfrey have led some herbalists to be wary of using it internally.
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It is used in herbal pastes, ointments, tinctures, decoctions, poultices and in cosmetics.
It is a popular addition to herbal salves and ointments, which can be used for bruises, sprains, eczema, swellings and burns.