efficacy

efficacy — noun

1. the degree to which a method, product, or approach succeeds at doing what it was

1.nounC1
Definition

the degree to which a method, product, or approach succeeds at doing what it was designed or expected to do

Example

The new teaching method's efficacy was clear when test scores rose across the school.

collocation: efficacy of [method/system/program]

Diya questioned the recycling system's efficacy after learning most waste went to landfill.

collocation: question the efficacy of

Common collocations
  • question the efficacy of
  • examine the efficacy of
  • doubt the efficacy of
  • prove the efficacy of
  • the efficacy of [something]
Synonyms
  • effectiveness

    the most common, everyday equivalent; 'efficacy' is more formal

  • utility

    focuses more on practical usefulness than ability to produce results

  • potency

    usually refers to strength of a chemical, drug, or argument; narrower scope

Antonyms

Grammar Patterns

the efficacy of [something]

Usage Note

Typically uncountable and used in formal or academic writing rather than everyday conversation. Often paired with a prepositional phrase beginning with 'of' to specify what is being evaluated.

Common Mistakes

We measured the efficiency of the new marketing campaign.' (when the question is whether it worked)
We measured the efficacy of the new marketing campaign.
'efficiency' is about doing things without waste; 'efficacy' is about producing the desired result.

2. a measure of how successfully a medical treatment, drug, or therapy performs whe

2.nounC2
Definition

a measure of how successfully a medical treatment, drug, or therapy performs when tested in a controlled research setting, before or after it reaches patients

Example

Clinical trials test the efficacy of new vaccines before they reach the public.

collocation: clinical efficacy

The drug showed high efficacy in treating the disease during the initial lab tests.

collocation: show high efficacy in treating

Common collocations
  • test the efficacy of
  • safety and efficacy
  • clinical efficacy
  • show high efficacy
  • proven efficacy
Synonyms
  • effectiveness

    used more broadly in everyday medical talk; 'efficacy' is the technical research term

  • potency

    emphasises the strength or power of a drug at a given dose, rather than overall success rate

Grammar Patterns

the efficacy of [drug/treatment/vaccine]

safety and efficacy

Usage Note

Frequently appears alongside 'safety' in the fixed phrase 'safety and efficacy'. Subject of evaluation is typically a treatment, drug, vaccine, or therapy. Common in regulatory and pharmaceutical contexts.

Common Mistakes

The drug showed high efficiency in the clinical trial.
The drug showed high efficacy in the clinical trial.
In medical and scientific contexts, 'efficacy' (not 'efficiency') is the correct term for how well a treatment works under controlled conditions.

Word family

  • efficacious
    adjectiveC2
  • inefficacious
    adjectiveC2
  • inefficacy
    nounC2

Synonym discussion

Sense: noun/1

Efficacy, effectiveness, efficiency, potency, and usefulness all relate to how well something works, but they differ in scope and context. Efficacy is the most formal term, used when evaluating whether something achieves its intended result, especially in academic or professional writing. Effectiveness is the everyday equivalent, appearing in both casual and formal contexts. Efficiency shifts the focus to how resources (time, money, energy) are used — a method can be effective without being efficient. Potency refers to strength or power, typically of drugs or arguments, while usefulness centres on practical value or convenience. For example: 'The drug's efficacy was confirmed in the trial, but its high cost raised questions about its efficiency in a public health system.'

effectiveness ↗efficiency ↗potencyusefulness

Common collocations

Nouns

Adverbs

Phrases

  • safety and efficacy
  • prove the efficacy of
  • test the efficacy of
  • question the efficacy of
  • demonstrate efficacy

Prepositions

Etymology

LatinFrenchEnglish

Efficacy comes from Latin 'efficacia', meaning 'the power to accomplish something'. This in turn comes from 'efficax' (effective, powerful), formed from the verb 'efficere' — 'to make happen, to accomplish' — which combines 'ex-' (out) and 'facere' (to do). The word entered English in the early 1500s from French, originally in medical and theological writing, and gradually spread to general formal use.