In the United States, there are many agencies and regulations which protect the rights and autonomy of clinical research subjects. The links below will lead you to these agencies and guidance documents.
All research involving human beings is evaluated for approval by an independent ethics board. They decide if the investigation is meritorious, and that the benefits are worth any risks. They also assure that the study design and procedures protect the rights of the volunteer. In all studies, the volunteer can stop participating without individual consequence.
Studies are monitored closely and if unexpected safety risks occur, they are often stopped.
Volunteers sign informed consent and are able to contact the ethics committee if they believe they have not been treated fairly.