WHAT IS EFFICACY?

Before defining efficacy – let us understand that there are couple of other terms which go hand in hand with the word ‘efficacy’ – they are ‘potency’ and ‘effectiveness’. 

• Say ’50 mg’ amount of a drug A is needed to alleviate a condition; now ‘100 mg’ amount of drug B is needed to alleviate the same condition.  Without any confusion we can say that drug A is more potent than drug B.

• Let us say, same amount of drug A and drug B are taken separately for a condition (by a patient with one and only one problem/condition, ideal subject). If drug A completes the job to alleviate the condition, however drug B partially alleviates the condition then we can say that drug A is more efficacious than drug B.  [Efficacy: maximum response achievable from a drug or to the capacity for sufficient therapeutic effect/beneficial change.  Therefore, efficacy trials determine whether an intervention produces the expected result under ideal circumstances]   

• Efficacious drugs cannot always be effective.  No confusion please.  Because effectiveness refers to ‘’how drug works in a real-world situation’. 

(1) Efficacious drug for a symptom should not have side effects and thus can become an effective drug.  For example, say we have a highly efficacious drug for certain condition – but it also has so many side effects.  Then we can say that this drug’s effectiveness is lower than its efficacy.

(2) in the real world, the drug administered to a patient has to interact with other medications taken by patient; depends on health conditions of the patient; patient might not follow the prescribed dose or duration by the physician etc.  Therefore effectiveness is always lower than efficacy.

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