I never have liked the term "guinea pig" when referring to human beings. To make matters worse, it is usually used in a negative way and has a negative connotation. Whether we like the term or not, the term "guinea pig" simply means "somebody or something experimented on: somebody or something used as the subject of an experiment, test, or trial" according to the Bing dictionary. It really doesn't have to be considered something negative; however, I would like to recommend that instead of using the term "guinea pig" instead we use the term "helpful pioneers"!
Pioneers paved the way for others to live a better life. They endured hardships to prevent future generations from having to do likewise. Pioneers were brave and faced uncertain futures. Don't you think that is a more fitting description of the good-hearted souls who take part in clinical studies to progress our current knowledge of whatever subject is being studied?