efficient - effective - efficacious


Senior Member
Good morning friends,

I've always had problems telling the exact difference between those three adjectives which obviously share a common etymology. I seem to often use one when I should use the other.
Could you help me?
  • sweetpotatoboy

    Senior Member
    English, UK (London)
    In theory, they all cover roughly the same possible meanings.

    However, I would make the following distinction:

    Effective: focuses on whether something achieves the required objective.

    Efficient: focuses on the ease/speed/convenience with which the objective is achieved.

    For example, something can be effective (it does the job) without being particularly efficient (e.g. it takes too long).

    Efficacious: I would avoid this altogether in most cases. It's not a commonly used or known word. (The noun 'efficacy' is somewhat more used, though.)


    Lapsed Moderator
    English-Ireland (top end)
    I can understand your problems - and you didn't mention effectual:)
    Like many others, these words have changed their meaning over the years.

    Efficient and effective, in particular, have suffered from being defined, used, abused and re-defined by management theorists seeking to use distinct terms for nuances of meaning. The dictionary definitions of each include the other, but each has a range of definitions not shared with the other.

    Efficacious - used to describe things, methods, processes - but not people. There is a strong association with cures, remedies, treatments. Whatever is efficacious will produce the required result.

    In despair, I opened Fowler ...
    Efficacious applies only to things (especially now to medicines) used for a purpose, and means sure to have, or usually having, the desired effect.
    Efficient applies to agents or their action or to instruments etc, and means capable of producing the desired effect, competent or equal to the task.
    Effectual applies to action apart from the agent, and means not falling short of the complete effect aimed at.
    Effective applies to the thing done or its doer as such, and means having a high degree of effect.
    I hope that helps. I'm not sure I understand all the nuances of meaning set out there - and this is from the second edition, not "New Fowler" so it comes from a time before the blossoming of management consultancy.

    Edit: ... and I wouldn't argue with sweetpotatoboy's definitions either. I'm sure there are many more.


    Senior Member
    Yes, it is very common to use the adverb efficaious in drugs.
    People talk about efficacy rate in pharmaceutical trials.


    New Member
    chinese, China
    I am lost! As a non-native english speaker, it is so hard to make a distinction among them. But the example of "something can be effective (it does the job) without being particularly efficient (e.g. it takes too long)." from sweetpotatoboy is very good and easier to understand than textbooks for me. The definitions from panjandrum is also very good, although i only got the meaning of "efficaious" :)


    Senior Member
    U.S. English
    From Zahra's English Book:

    If something is effective it (1) has an noticeable effect: The actor made a most effective entrance [or (2) coming into effect: effective November 1, 2006.] If it is effectual it produces a particular effect, usually the one intended: We took effectual steps to redress the situation. If a thing is efficacious it has the power or potential to produce a particular effect: These tablets are efficacious against malaria. If a thing is efficient it works well: Josie did a very efficient job with the lawns.
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