Cool MateEfficacy vs Success
When to use each in English, with meaning, register, and examples.
Efficacy
FormalBeyond 10,000 (less common)
Success
Top 1,000 (very common)A1noun
Most formal: EfficacyMost common: Success
| Efficacy | Success | |
|---|---|---|
| Pronunciation | 🇬🇧 //ˈɛfɪkəsi//🇺🇸 //ˈɛfɪkəsi// | 🇬🇧 /["/səkˈses/"]/🇺🇸 /["/səkˈses/"]/ |
| Meaning | Effectiveness or how well something works. | When you achieve something you wanted to do. |
| Example | The efficacy of the new vaccine was confirmed in clinical trials. | His success in the competition made him very proud. |
| Register | Formal | Neutral |
| How common | Beyond 10,000 (less common) | Top 1,000 (very common) |
| CEFR level | - | A1 |
| Part of speech | noun | |
| Collocations | clinical efficacy, evaluate efficacy, prove efficacy, demonstrate efficacy, measure efficacy | enormous, great, immense, amount, degree, level, achieve, attain, enjoy, come, lie (in something), depend on something, rate, story, factor, with success, without success, success in, a chance of success, a hope of success, confident of success, resounding, roaring, rousing, be, prove, have, wish somebody (every) success |
| Antonyms | - | failure, defeat, setback |
| Common mistakes | Confused with 'efficiency', which refers to doing things quickly and without waste., Often used without proper context; needs to specify what is being evaluated., Mispronounced, especially by non-native speakers. | Confused with 'successfully', which is an adverb., Used incorrectly as a verb instead of the noun form., Misplaced in sentences, such as 'We have success' instead of 'We are successful'. |
| Usage notes | Used primarily in formal contexts like research, medicine, and discussions about policy. Less common in everyday conversation. | Use 'success' when talking about achieving goals or winning in a friendly context. Avoid using it in negative situations or as a mockery. |
Frequently asked questions: Efficacy vs Success
What's the difference between Efficacy and Success?
Efficacy: Effectiveness or how well something works. Success: When you achieve something you wanted to do.
Which is more formal: Efficacy and Success?
Efficacy is the most formal of these.
Which is more common: Efficacy and Success?
Success is the most common in everyday English.
Can I use Efficacy and Success interchangeably?
Not always. Efficacy and Success are related and overlap in some contexts, but they differ in register, how common they are, and usage, so swapping one for another can change the meaning or tone. Check the differences above before substituting.