It is important to monitor drug therapy because the effects of a drug can vary significantly between individuals. Wherever possible, therapeutic effect should be monitored using a clinical endpoint (i.e. a measure that directly reflects how the patient feels, functions or survives). In practice, it is often not feasible to use a clinical endpoint to guide therapy, particularly for preventive treatments. The next best option is to use a surrogate endpoint: a measure that predicts whether the clinical endpoint will be achieved.