A randomized trial is a randomized trial. The basic ingredients do not change with different purposes whether for treatment or prevention of disease. Likewise, the problems and difficulties are mostly the same. But there are differences in approach and philosophy depending on whether for treatment or prevention of disease. One thing setting prevention trials apart from treatment trials is the risk-benefit calculus of the two classes of trials. Treatment trials are undertaken to 'cure' or ameliorate disease, whereas, prevention trials are undertaken in the hope of preventing or delaying onset of disease. The risks of harm in treatment trials is contemporaneous with prospects for benefit making the calculus reasonably straightforward. But that time relationship does not exist in long-term drug prevention trials where the risks from treatment start accruing on initiation of treatment, but where the prospect of benefit is down the road and comes, if all, in the form of disease avoided. This separation of risk versus benefit makes for difficult decisions as to how long to continue a trial in the absence of a difference in the test-assigned versus the control-assigned group. Other differences relate to choice of study treatments, choice of outcome measure, approach to recruitment and age cutoffs, and issues related to monitoring.
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