Supporters of the Bracebridge nurse presented a bill to the Ontario parliament in 1938 to allow Caisse to treat cancer patients with Essiac free from the constant threat of arrest to which she had been subjected. Over 55,000 people signed a petition supporting the bill, including patients, their families, and many doctors. The bill failed to pass by three votes.

This set the stage for the creation of the Royal Cancer Commission, which many believed was a judicial farce. Comprised of six orthodox physicians with expertise in surgery, radiation, and diagnostics and led by an Ontario Supreme Court justice, the commission was charged with an impartial investigation of alternative cancer therapies. Public hearings opened in March 1939.

Even though 387 of Caisse's patients showed up to testify, only 49 were allowed to be heard. One after another, patients and ex-patients testified that Rene Caisse had restored them to health and saved their lives after they had been given up as dead by their orthodox doctors.

Annie Bonar testified that her diagnosed uterine and bowel cancer had spread after radium treatments until her arm had swelled to double its size and turned black. Weighing ninety pounds the night before she was to have the arm amputated, she opted for Essiac therapy instead. After four months of the herbal treatment, her arm was back to normal and she had gained sixty pounds. A series of X-ray exams revealed she was cancer-free. The Royal Commission, however, listed Annie Bonar's case as "recovery due to radiation."

Walter Hampson, another patient of Caisse who testified, had cancer of the lip, diagnosed by a pathologist. Refusing radium, he underwent Essiac therapy and was restored to normal. Despite the fact that he had never had an operation (other than the removal of a tiny nodule for analysis), the commission classified his case as "recovery due to surgery." These examples could be multiplied many times.

In addition to misattributing recoveries, the Royal Commission also labeled numerous cases as "misdiagnoses," even though the patients had been diagnosed as definitely having cancer by two or more qualified physicians. Using duplicitous tactics like these, the commission was able to conclude that "the evidence adduced does not justify any favourable conclusion as to the merits of 'Essiac' as a remedy for cancer...."

In 1942, a disheartened Rene Caisse, fearing imprisonment due to her medical work, closed her clinic. Over the next thirty-odd years, she continued to treat cancer patients in great secrecy from her home. Documents indicate that she was under surveillance by Canada's Health Department during the 1950s.

At the age of seventy, in 1959, Caisse was invited to the Brusch Medical Center in Massachusetts, where she treated terminal cancer patients and laboratory mice with Essiac under the supervision of eighteen doctors. After three months, Dr. Charles Brusch, eminent physician to the New England elite, and his research director, Dr. Charles McClure, concluded that Essiac "has been shown to cause a decided recession of the mass, and a definite change in cell formation" in mice. "Clinically, on patients suffering from pathologically proven cancer, it reduces pain and causes a recession in the growth; patients have gained weight and shown an improvement in their general health.... Remarkably beneficial results were obtained even on those cases at the 'end of the road' where it proved to prolong life and the quality of that life.... The doctors do not say that Essiac is a cure, but they do say it is of benefit."