Essiac/Flor Essence (PDQ®): Complementary and alternative medicine - Health Professional Information [NCI] - Laboratory / Animal / Preclinical Studies
In 2004, a mixture of the Essiac herbs showed a decreased proliferation in a prostate cancer cell line. Results of laboratory (in vitro) studies of Essiac have been reported. Reviewed in 
In the mid-1970s, the developer submitted both dried and liquid samples of Essiac to the Memorial Sloan-Kettering Cancer Center (MSKCC) in New York City for evaluation of its immunotherapeutic and chemotherapeutic potential. Reviewed in  No immunostimulatory or chemotherapeutic activity was detected in eight animal experiments that utilized the S-180 mouse sarcoma tumor model.
In the early 1980s, the corporation that acquired the four-herb recipe for Essiac from the developer submitted another sample to the MSKCC for evaluation in additional animal studies. No anticancer activity was detected in 17 separate experiments that utilized a variety of animal leukemia and tumor models. Reviewed in 
In 1983, the National Cancer Institute tested a liquid sample of Essiac that was provided by the manufacturer after the Canadian Department of National Health and Welfare (Health Protection Branch) requested that it be tested in animals. Reviewed in  These studies revealed no anticancer activity in the mouse P388 lymphocytic leukemia tumor system and found lethal toxicity at the highest concentrations of Essiac administered to test animals. It is not known, however, how the concentrations used in these animal tests compare with the concentrations achieved in humans after the consumption of the manufacturer's recommended doses.
There are conflicting results in the peer-reviewed literature. One study suggests that Flor Essence enhances tumor growth in vitro, a finding that is contradictory to the widely available anecdotal evidence that this product suppresses or inhibits tumor development. Another study suggests that the growth of human breast cancer cells is stimulated through estrogen receptor (ER)–mediated as well as ER–independent mechanisms of action from Flor Essence and Essiac herbal tonics. A third study demonstrated antiproliferative and differentiation-inducing properties in vitro only in high concentrations of Essiac and Flor Essence herbal teas.
The 2004 in vivo study of Flor Essence in a rat model looked at mammary tumor development following administration of the herbal compound. Sprague-Dawley rats (N = 112) were assigned to one of three groups. The control group (n = 35) received water only. The second group (n = 40) received 3% Flor Essence in their drinking water in an attempt to provide a dose equivalent to that recommended in the popular literature. The third group (n = 37) received 6% Flor Essence in their drinking water to investigate the dose-response relationship. Mammary tumors were induced by a 40 mg/kg/bw dose of 7,12-dimethylbenz(a)anthracene. At 19 weeks, palpable mammary tumor incidence was greater (65% and 59.4%) in both Flor Essence groups compared with controls (51%). Terminal necropsy was performed at age 23 weeks or when tumor burden became too great. Results showed mammary tumor incidence was 82.5% for controls compared with 90% and 97.3%, respectively, for rats consuming 3% and 6% Flor Essence.