Repeat participation among normal healthy research volunteers - professional guinea pigs in clinical trials?

The recent death of a normal healthy volunteer, as well as the increased use of normal volunteers as research subjects, has heightened the attention given to the participation of normal volunteers in clinical research. An overlooked sub-population of normal healthy volunteers are repeat, veteran volunteers. This essay discusses ethical and methodological issues associated with the use of repeat volunteers in research, along with existing guidelines regarding the use of repeat healthy volunteers, and concludes with recommendations for safeguarding repeat volunteers and ideas supporting centralized recruiting.