A drug can do exactly what it is supposed to do in a laboratory and still fail in the real world. Efficacy is the measure of how well a treatment works under ideal conditions, usually in a controlled clinical trial. Effectiveness is how well it works in ordinary practice, where patients miss doses, have other conditions, and do not fit the strict criteria of a study. The two can differ by a wide margin.

Efficacy is expressed in different ways depending on the disease. In cancer, it might be tumor shrinkage or overall survival. In hypertension, it is millimeters of mercury dropped. In depression, it is a change on a rating scale. Researchers use statistical tests to determine whether the observed effect is real or due to chance. They also look at the number needed to treat, which is how many patients must receive the drug for one to benefit. A drug with a small absolute effect can still be useful if the condition is common and the side effects are mild.

Key concepts:

• Efficacy. Works under ideal conditions, like a clinical trial.
• Effectiveness. Works in real-world practice.
• Number needed to treat. How many patients need the drug for one to benefit.
• Statistical significance. The result is unlikely due to chance.
• Clinical significance. The result matters to patients.

A drug can be statistically significant but not clinically meaningful. Regulators and clinicians weigh both.