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.Flor Essence
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.The Individual Herbs of Essiac and Flor Essence
Laboratory and animal experiments have shown that some of the chemicals in the herbs used to make Essiac and Flor Essence have antioxidant, anti-inflammatory, estrogenic, or anticancer activity. Reviewed in [7-15]
Among the herbs used in both mixtures, burdock root (Arctium lappa L.) contains several flavonoids and polyphenols that have shown antioxidant activity; Indian rhubarb root (Rheum palmatum L.) contains several anthraquinones, including emodin and aloe-emodin, which have demonstrated anti-inflammatory and cytotoxic effects; sheep sorrel (Rumex acetosella L.) contains several types of anthraquinones, including emodin and aloe-emodin, as well as phytoestrogens, which may possess both procancer and anticancer activity; and slippery elm bark (Ulmus fulva Michx.) has been shown to contain antioxidants. Reviewed in [7-13]
Among the herbs found in Flor Essence alone, watercress (Nasturtium officinale R.Br.) contains phenethyl isothiocyanate (PEITC), which has shown cytotoxic and antitumor activities; blessed thistle (Cnicus benedictus L.) contains cnicin, which is a sesquiterpene lactone that has demonstrated cytotoxic, antitumor, and anti-inflammatory effects, and arctiin and arctigenin, which are lignans that have shown anticancer activity; red clover (Trifolium pratense L.) contains a complex mixture of phytoestrogens including genistein, which has demonstrated antiangiogenic, estrogenic, and procancer and anticancer effects (depending on the dose); and extracts of kelp (Laminaria digitata [Hudson] Lamx.) have shown immunostimulatory and antitumor activities. Reviewed in [7,8,11,12,15]
Whether equivalent concentrations of relevant molecules can be achieved in the bloodstream of individuals who consume Essiac or Flor Essence in the amounts recommended by their manufacturers has not been determined. An uncharacterized Flor Essence commercial product was dosed at amounts less than those recommended by the manufacturers for humans, and there was an increase in tumor incidence in this model.
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- Bennett LM, Montgomery JL, Steinberg SM, et al.: Flor-Essence herbal tonic does not inhibit mammary tumor development in Sprague Dawley rats. Breast Cancer Res Treat 88 (1): 87-93, 2004. [PUBMED Abstract]
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