How do researchers make sure clinical study participants are safe?

It's a good idea to ask questions and gather information before you make a decision about participating in research. It's also important to remember that strict rules protect participants in research studies. Researchers take these rules very seriously because they care about participants health. Additionally, researchers must follow the rules if they want to be allowed to do more research. Below is some more information about how study participants are kept safe.
Federal agencies: These groups protect the rights and well-being of clinical research participants. Federal agencies including the FDA oversee much of the medical research in the United States.
Informed consent: The process of learning about a clinical research study before you decide if you want to participate.
Institutional review boards (IRBs): IRBs oversee the research centers where clinical studies are conducted. IRBs review and approve study protocols (plans) for scientific merit, participant safety, and ethical considerations. Participants who have questions may contact the IRBs at the research center they are considering. IRB members usually include scientific experts as well as community members.
Leaving the trial: Participants who wish to leave the trial because they feel unsafe may do so without penalty
Peer review: Federal agencies may review individuals and institutions conducting research
Primary health care: If you participate in a study, talk with your regular doctor to be sure that medications you normally take are safe to use with any medications you may take for the study. Sometimes the researcher will ask for permission to speak with your regular doctors to make sure you get the best treatment.
Protocol: A plan that details what researchers will do during a study. Every study must have a protocol. The protocol includes information on how researchers will work to keep participants safe.
This answer is based on source information from National Institute of Mental Health.

Some research centers offer study participants access to a Research Subject Advocate. This person is usually someone with extensive training in research ethics and research subject safety and he/she is available when there are concerns about matters such as participant welfare, informed consent, conflict of interest, etc.