Placebos are rarely used in cancer treatment clinical trials. They are used when there is no standard treatment. Or, they may be used in a clinical trial that compares standard treatment plus a placebo, with standard treatment plus a new treatment. The placebo is designed to look like the medicine being tested, but it is not active. Using a placebo in this way can help prevent patients and their doctors from figuring out which treatment group they were assigned to.
Placebos are another way to help prevent bias in research. You always will be told if the study uses a placebo.