Concepts of 'Efficacy'

In medicine, there are a number of terms when describing efficacy. These are outlined as follows:

  • Efficacy is the extent to which a desired effect is achieved. Efficacy measures how well a treatment works in clinical trials or laboratory studies.

  • Effectiveness relates to how well a treatment works in practice after a medicine is generally available. The word ‘effective’ is sometimes used in a qualitative way, e.g. ‘being very effective’ or ‘not very effective’.  

The benefit monitored depends on the type of medicine. For example, the benefit of taking a medicine to lower blood pressure is to prevent a stroke and eventual death. The benefit of taking allergy tablets is to prevent a rash. From this we can say that the efficacy of a medicine is evaluated in relation to the medical problem it is intended to treat – this is called the ‘indication’. It cannot be assumed that if a medicine produces the desired outcome (efficacy) for one condition, that it will do the same if the medicine is used for a second condition. Let’s look at a case example:

Example 1
Originally, Viagra™ (sildenafil) was tested as a treatment for high blood pressure (hypertension) and ‘angina pectoris’ (a symptom related to heart disease). Viagra™ went to Phase I and II clinical trials for the indication of angina pectoris, where it did not show any promising results. However, during the clinical trials it was discovered that Viagra brought about penile erections. As a result, the company wanted to market Viagra™ for a new indication of erectile dysfunction. They were required to submit further data to show the efficacy of the medicine in relation to the new indication, in an appropriate population. 

Efficacy and population

Lately, the importance of testing new medicines in various populations such as different genders, ethnic groups and age groups has been debated. This is because a medicine can only be considered efficacious based on the population(s) included in the clinical trial(s). The efficacy of a medicine cannot automatically be extended to populations that were not included in the clinical trial(s).

Efficacy and conditions of use

The conditions of use for a medicine also have a great impact on efficacy. For a medicine to have maximum effect, it must be taken at the specific time and in the specific dose. If not, the efficacy may be affected.

Efficacy and interactions

A medicine should not be taken with other medicines and/or foods for which a negative interaction is known. This might have an impact on both efficacy and safety of the medicine.