These terms have acquired particular meanings – more limited than their everyday meanings – in the context of therapeutic trials.Pharmacological effects of drugs (i.e. their effects on cells, organs and systems) are, in principle, simple to measure in animals, and often also in humans. We can measure effects on blood pressure, plasma cholesterol concentration, cognitive function, etc., without difficulty. Such measures enable us to describe quantitatively the pharmacological properties of drugs, but say nothing about their usefulness as therapeutic agents.Efficacy describes the ability of a drug to produce a desired therapeutic effect in patients under carefully controlled conditions. The gold standard for measurements of efficacy is the randomized controlled clinical trial, described in more detail in Chapter 17. The aim is to discover whether, based on a strictly defined outcome measure, the drug is more or less beneficial than a standard treatment or placebo, in a selected group of patients, under conditions which ensure that the patients actually receive the drug in the specified dose.