The two most generally accepted methods to prove the safety of a generic version of a drug are to either repeat most of the chemistry, animal and human studies originally done, or to show that the drug performs comparably with the original brand name drug. This second option is called a "comparative bioavailability" study. During this type of study, volunteers are given the original drug, and then separately later the generic drug. The rates at which the drug is delivered to the patient (into their blood stream or otherwise absorbed) are measured to ensure they are the same. Because the same active ingredient is used the major concern is just that it delivers the common chemical(s) at the same rate so that they have the same effect.