Randomised trials are crucial for establishing the effectiveness of new drugs and procedures.  However, they are less effective at detecting uncommon but clinically significant side effects.  The authors propose a solution.  All UK general practices could be randomized to be allowed to prescribe new licenced drugs earlier or later.  This would produce a large pragmatic cluster trial which could enable rare, but harmful, effects to be demonstrated more quickly than the current usual practice of looking for harmful events in observational datasets.  Given current computerisation of practice records such an approach is feasible and likely to be cost-effective.