Participating in multiple studies
Sometimes volunteers participate in multiple studies. Dutch research centres that investigate medicinal products calculate when a volunteer may participate in a next study. To calculate this, they look back four studies. Research centres that carry out a lot of research with medicinal products using healthy volunteers check this using a system called Verified Clinical Trials (VCT). VCT records who participated in which study and when.
People are not prohibited to participate in multiple studies at the same time. However, this is not the preferred approach because it can have a detrimental effect on your safety and the reliability of the research. Participating in one research study is not allowed to affect another.
Medicinal products
In the case of research with a medicinal product you are usually not allowed to participate in multiple studies at the same time. If you do there will be a risk of cross reactions which means that one medicinal product interferes with the working of another.
Break
Sometimes people are obliged to take a break between studies. That is certainly necessary if there is a possibility of the studies affecting each other. How long this break has to be depends on the type of research. You should ask the investigator whether you are allowed to participate in new research (again).