In 1922, when a patient claimed her breast cancer had been cured by an Ojibwa medicine man, Rene Caisse, a public health nurse from Ontario, Canada became interested in learning the herbal formula for the tea. After obtaining the recipe, she is reported to have successfully cured her aunt's stomach cancer. Two years later, Caisse opened a cancer clinic and began treating thousands of patients with the tea, which she named Essiac, Caisse's name spelled backwards.

In 1977, one year before her death, Cassie gave the formula for the tea to a Toronto manufacturer, with a promise that the tea be sold at an affordable price to cancer patients. The original formula for Essiac included burdock root, slippery elm inner bark, sheep sorrel and Indian rhubarb root. Watercress, blessed thistle, red clover, and kelp were later additions to the formula and sold as Flor Essence.
In 1938, Canadian medical authorities investigated Cassie's clinic and found little evidence to support Cassie's curative claims regarding Essiac. In 1959, and again in the 1970's, Memorial Sloan-Kettering Cancer Center tested Essiac, but found no evidence in its ability to affect cancer. In 1983, the Canadian federal health officials asked the US National Cancer Institute, NCI, to conduct Essiac tests, and the NCI found the tea did not have cancer curative ability.

According to NCI, while there have been no published clinical trials showing the effectiveness of Essiac in the treatment of cancer, some of the specific herbs contained in the mixture have shown some anti-cancer effects in laboratory experiments. However, there is no scientific evidence to support its use for the treatment of cancer in humans.

Over the years, there have been many testimonials claiming Essiac has cancer curative properties. Essiac is said to strengthen the immune system, improve well being, relieve pain, increase appetite, reduce tumor size, cleanse the blood, promote cell repair, restore energy levels, relieve inflammation, lubricate bones and joints, stimulate the stomach, and eliminate excess mucous in organs, tissues, lymph glands, and nerve channels. Originally, the explanation for how Essiac worked, was that it could change tumors into normal tissue.

At the time NCI researchers conducted its testing of Essiac, they did note that there were few serious side effects, however, and that people may have derived psychological benefits of well-being from the herbal tea. In rare cases, the Essiac tea formula may cause headache, nausea, diarrhea or constipation, vomiting, low blood sugar, liver damage and kidney damage.

So does it work? According to scientific research, no, but the no is followed by an admission that some of the herbs in the Essiac formula do have anti-tumor properties. According to testimonials, there are patients who swear it cured them of cancer. Of course, those who market Essiac, are going to tell you that it indeed does have curative powers. The Essiac formula is easy to obtain, and the ingredients easy to purchase, and it is relatively easy to make, if the tea is something that sounds of interest to you. To date, there have not been any major clinical trials to test the efficiency of Essiac tea, or Nurse Rene Cassie's claims. Also, there is no way of verifying if the formula available today, is the same formula once used the Ojibwa shaman healer, to cure cancer, nearly 90 years ago.