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Efficacy vs. Efficiency

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  Angbeen Chaudhary  —  Grammar Tips
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The English language has a lot of similar words amongst its ranks. But while many of these words may be similar to one another, they are not the same. They often carry slightly different meanings or they place a different emphasis on the words around them. Such is the case with our two words today, efficacy vs. efficiency.

In today’s post, I want to go over the differences between these two words. I will talk about their definitions and their functions in a sentence. I will also provide example sentences, so you can see how they work for yourselves.


The word efficacy’s origin dates back to early 16th century: from Latin efficacia, from efficax, efficacy. The word efficiency also originated in late 16th century (in the sense ‘the fact of being an efficient cause’): from Latin efficientia, from efficere ‘accomplish’

Efficacy as noun:

The word efficacy is used as a noun in English language where it means the ability to produce a desired or intended result.

There is little information on the efficacy of this treatment.

Efficiency as noun:

The word efficiency is also used as a noun in English language where it means the state or quality of being efficient.

This machine has greater energy efficiency than the previous one.

An action designed to achieve efficiency is also termed so.

The reforms will lead to efficiencies and savings.


Efficacy vs. Efficiency

The efficacy of this diet regimen is still debated.

Critics questioned the efficacy of the proposed regulations in stopping crimes.

The general spoke about the efficacy of the bombing three days after the raid.

There is in still limited information on the efficacy of this cancer treatment.

The efficacy of this drug is increased when taken with a meal.

Your branch has greatly improved its efficiency last quarter.

We have implemented new technologies that will greatly increase our efficiency.

My new car has a much higher fuel efficiency rating than my old one.

Efficiency or efficacy:

While these two words are similar, efficiency vs. efficacy have different meanings. Efficacy is about whether or not something can be done at all. Efficiency is about how something is done and if it minimizes waste. In case you are wondering how you’ll remember all of this next time you go to use efficiency or efficacy, here is a neat trick to remember the difference. Efficacy has to do with the capacity to do something. Both efficacy and capacity have the letters “CA” right next to each other. Efficacy also has to do with ability, both of which have the letter “A” in them. When thinking about efficacy, think about capacity and ability. Efficiency has to do with minimizing waste. Something that has a perfect level of efficiency will have no waste. Both efficiency and no have the letter “N” in them. When thinking about efficiency, think no waste.


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