Forget the kidneys.... Supplements with creatine or androstenedione are suspect to cause testicular cancer. This came to light as there was a marked incidence of 35 year olds presenting with testicular cancer. The link - they were mostly gym buffs who took supplements containing these. If you Google it possibly you will get that this is not true as obviously the supplement manufacturers will write the opposite. The cigarette companies do exactly the same.
It's up to you to decide whether your balls - or your muscles - are an asset.