The concept of drug class effect is profoundly rooted in clinical practice. The use of drugs seen as similar in their clinical effects--and therefore interchangeable--is very frequent: two examples of this are the use of beta-blockers in arterial hypertension and angiotensin-converting enzyme (ACE) inhibitors in congestive heart failure. The definition of drug class effect is based on three concepts: a similar chemical structure (for example, the dihydropyridine ring of some calcium channel blockers), a similar mechanism of action (beta-blockers block adrenoreceptors), or similar pharmacological effects (antihypertensives, antianginals, etc.). In this article, we will describe the type of evidence that a cardiologist can use in order to select a specific drug (from within a class). It constitutes a clinical approach, different from the one that might be used by a third party payer (more interested in cost-effectiveness issues) or the pharmaceutical industry (more interested in promoting sales). As usual, the recommendations are based on the strength of scientific evidence.