Rational prescribing: the principles of drug selection and assessment of efficacy.

Most experienced prescribers believe that the principles of rational prescribing underpin their choice of therapy. However, without a robust understanding of the basis for decisions, drug choice can become formulaic and ignore individual patient variability. Rational prescribing should: • maximise effectiveness • minimise risks • respect patient choice, and • minimise costs. A logical sequence of events usually precedes a rational prescription, comprising: • making a diagnosis (or differential diagnoses) • determining the prognosis of the condition to be treated • determining the goal(s) of treatment (eg curative, symptom-relieving, preventive or, occasionally, an aid to the diagnostic process), and • selecting an appropriate type of treatment. It is unusual for only one treatment option to be available and a selection is made from a range of possible approaches. For example, an explanation may be sufficient to satisfy a patient with brief, infrequent episodes of supraventricular tachycardia. For longer episodes, vagotonic manoeuvres may terminate the symptoms. More frequent or symptomatic tachycardias may be suppressed by a beta-blocker or calcium channel antagonist, and it may also be appropriate to consider an electrophysiological study with a view to ablation of the originating focus. Evidence from large-scale randomised controlled trials (RCTs) is often used to guide the choice of treatment. However, many assumptions are made when extrapolating evidence from a highly selected population sample to general clinical practice, ignoring exclusion criteria that were applied during recruitment. A drug that is superior in clinical trials may prove less effective in clinical practice because of variability in individual responses or be less suitable for an individual because of potential adverse effects. In the former situation, there is no reliable way to determine the probability of a successful outcome in an individual; in the latter, good clinical judgement should override the results of clinical trials.