Can you leave a trial at any time
Yes. Just as you can refuse to join a study, you may leave a study at any time. Your rights as an individual do not change because you are a patient in a clinical trial. You may choose to take part or not, and you can always change your mind later, even after you enter a trial.
You may also refuse to take part in any aspect of the research. If you have questions at any time about any part of the study, be sure to ask your doctors. If you are not satisfied with the answers, you may consider leaving the study. If you decide to leave, it will not be held against you. You can freely discuss other possible treatments and care with your doctors and nurses.
The above information is taken from "What Are Clinical Trials All About?" published by the National Institutes of Health, National Cancer Institute