Efficacy vs. Efficiency: What’s the Difference?

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.

What is the Difference Between Efficacy and Efficiency?

Should you use efficacy or efficiency in a sentence?

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.

After reading this post, you should never again confuse these two words in your writing.

When to Use Efficacy

efficacy versus efficiency grammar rulesEfficacy is a noun and is defined as the power or capacity to produce a desired effect.

  • 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.

Efficacy is widely used, among other places, in the medical field to refer to medical treatments.

  • 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.

The word efficacy has to do with the ability or capacity to do something, but not about how something is done.

That is where efficiency comes in.

When to Use Efficiency

define efficacy v efficiency meaningEfficiency is also a noun and is defined as the quality or property of being efficient. Efficient means acting or producing with a minimum or waste, expense, or unnecessary effort.

  • 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.

While efficacy has to do with whether or not something is able to be done at all, efficiency has to do with how something is done and whether or not it is done without much waste, effort, or time.

Adjective Forms of Efficacy and Efficiency

As mentioned above, both of these words are nouns, but they do both have adjectival forms.

The adjectival form of efficacy is efficacious.

  • This is a very efficacious diet regime.
  • Daily exercise is one of the most efficacious ways to stay healthy.

The adjectival form of efficiency is efficient.

  • This is a very efficient car.
  • This month we need to focus on becoming more efficient.

Tricks to Remember the Difference

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.


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.