In 1924, Caisse's aunt, Mireza Potvin, was diagnosed with advanced cancer of the stomach and was told she had six months at the most to live. Remembering the Indian brew, Rene asked her aunt's physician, Dr. R. O. Fisher of Toronto, for permission to try it on her dying relative. Dr. Fisher consented, and Rene gathered the herbs to brew the tea. After drinking the herbal concoction daily for two months, Mireza Potvin rallied, got well, and went on to live another twenty-one years.

Soon Caisse and Dr. Fisher teamed to treat cancer patients who had been written off by their doctors as terminal. Many of these patients, too, showed dramatic improvement. Working nights and weekends in Toronto in her mother's basement, which Rene had converted into a laboratory, she and Dr. Fisher experimented on mice inoculated with human cancer. They modified the combination of herbs to maximize efficacy. It was at this point that Rene named the herbal treatment Essiac (her name spelled backwards).

One of Rene's first cases was a woman who had cancer of the bowel complicated by diabetes. In order to avoid further problems, the patient stopped taking insulin in 1925. Under Essiac therapy, the woman's tumor at first became larger and harder, almost obstructing her bowel. Then, as she continued her Essiac injections, the tumor softened, got smaller, and disappeared. Oddly enough, the woman's diabetes also disappeared during the course of Essiac treatment.

Dr. Frederick Banting, world-famous as the codiscoverer of insulin, reviewed this case in 1926. According to Caisse, Dr. Banting concluded that Essiac must have somehow stimulated the pancreatic gland into normal functioning, thus clearing up the diabetic condition. If this reported result is true, Essiac would appear to have potential in the treatment of diabetes.

Nine doctors petitioned the Canadian federal health department in 1926, urging that Caisse be allowed to test her cancer remedy on a broad scale. In their signed petition, they testified that Essiac reduced tumor size, prolonged life in hopeless cases, and showed "remarkably beneficial results," even where "everything else had been tried without effect."

In response, Ottawa's Department of Health and Welfare sent two investigating doctors armed with official papers to arrest Nurse Caisse or restrain her from practicing medicine without a license. When Rene explained to them that she was treating only terminal cases and accepting only voluntary contributions, the two interrogators backed off. One of them, Dr. W. C. Arnold, was so impressed by Caisse's clinical reports that he persuaded her to continue her experiments with mice at the Christie Street Hospital in Toronto. In that series of tests, mice implanted with human cancer responded to Essiac injections by living longer, their tumors regressing.

In 1935, the Town Council of Bracebridge turned over to Rene Caisse-for one dollar-per-month rent-the old British Lion Hotel for use as a cancer clinic. Over the next seven years, Caisse treated thousands of patients in this building, which had been repossessed by the village for back taxes. This unique arrangement came about after Dr. A. F. Bastedo of Bracebridge referred a terminally ill patient with bowel cancer to Caisse. Dr. Bastedo was so impressed by the patient's recovery, he persuaded the town council to make the hotel building available to Rene.

Shortly after the clinic opened, Caisse's seventy-two-year-old mother, Friselde, was diagnosed with cancer of the liver, inoperable because of her weak heart. One of Ontario's top specialists, Dr. Roscoe Graham, said she had only days to live. Rene began giving daily injections of Essiac to her mother, who had not been told she had cancer. After ten days of treatment, Friselde Caisse began to recover. She regained her full health, with diminishing doses of Essiac, and lived another eighteen years before passing away quietly from heart disease.