You only officially become a research subject if you have signed the declaration of consent. You can only do so in writing.
The researcher will give you a declaration of consent (also referred to as 'informed consent'). This declaration has to comply with strict requirements. It stipulates which research is involved and what you are giving permission for. Your declaration is valid once you have dated and signed it. You will be given a copy. Now you will be an official research subject.
Sometimes the researcher will want to inform your GP or specialist that you are participating in the research. This must then be stated in the declaration of consent. The researcher is only allowed to do so with your permission.
Sometimes the researcher will actually be obliged to inform your GP or specialist. In such instances you can only participate in the research if you have given your consent.
Sometimes a medical examination will be carried out first. That will only be possible if you have signed the declaration of consent. You can only participate in the research after you have come through the examination successfully.
If you change your mind, you always have the right not to participate. Or to stop during the research.