Before you enroll in a study you will be given a consent form to read and sign. This form gives you all the information about the clinical trial you are considering. Be sure that you have read the consent form thoroughly. Some people read it two or three times. If something is not clear on the consent form, or you want additional information, be sure to ask all of your questions to the study doctor or the study coordinator, whoever reviews the consent form with you.
Every clinical trial is different, and the benefits and risks of participating vary.
The consent form is your best source of information. Although previously you might have talked to the study doctor or study nurse, you might read the consent form and find out something about the clinical trial that you didn't expect. Perhaps it never occurred to you to ask about the risks of a certain procedure, or a placebo period. Maybe you didn't know that there would be an overnight study visit, and maybe any one of these details is enough for you to decide that you don't want to join after all. You can thank the study staff for their time and efforts, but at the same time politely decline to sign the consent form and leave the research center.