Carcinogens correlation studies involving

Long-term studies in animals have shown that comfrey is carcinogenic (USP, 1998). This carcinogenicity has been associated with the PAs found in comfrey. Several studies involving rats have made this correlation. 

In general, short-term tests have limitations. They do not include variables that animal studies do. It becomes difficult to use mathematical models to extrapolate test results to humans. For example, one of the notable short-term tests for carcinogens is the Ames test. The procedure uses Salmonella and the results involve dose, but not time or duration of exposure. Some people criticize the Ames test for a lack of reliability. The Ames test demonstrates high correlations between certain known carcinogens and human experience. For other carcinogens there are low correlations between test results and human experience. 

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