How to use the various Japanese words involving effect


効果, 有効, 効き目, 効力, 効能, 影響, and 作用 all have meanings than can roughly be
translated to "effect" or some variation of the word.

My understanding is:

  * 影響 and 作用 are the only ones that aren't strictly positive sounding
  * 作用 is typically used for natural/scientific effects, like chemical reactions or graviational laws
  * 効果 and 効能 are more like "effect" while 有効, 効き目, and 効力 are more like "effective"

But beyond this, I have a difficult time understanding when to use them
properly.




Your understanding seems fine.

  * **効能** [noun]: Refers to positive/expected effects of a therapy/medicine/onsen/etc.
  * **効き目** [noun]: Almost the same as 効能, but it is a wago (see [wago-and-kango](/questions/tagged/wago-and-kango "show questions tagged 'wago-and-kango'")) and tends to be avoided in academic contexts.
  * **効力** [noun]: Can be used in the same way as 効能 in therapeutic contexts, but mainly used in other contexts in the sense of validness/activeness, as in "the law is in _effect_ ", "the rule took _effect_ ".
  * **効果** [noun]: Almost the same as 効能 in therapeutic contexts. Also used in other contexts in the sense of illusion/tricks/decoration, as in "visual/audio _effect_ ".
  * **作用** [noun, suru-verb]: Tends to refer to direct effects at a more mechanical level (physics, physiology, etc). This is a neutral word that does not assume the effect is desirable. 副作用 ("side effect") is an important derivative word.
  * **影響** [noun, suru-verb]: Closer to "influence" rather than "effect".
  * **有効** [na-adj]: "Effective". The only na-adjective among these.

So "This drug is effective" can be translated as この薬には効能がある, この薬には効き目がある,
この薬には効果がある and この薬は有効だ. この薬には作用がある makes little sense unless this was said as
opposed to placebo.

Related:

  * [Difference between 影響 and 効果](https://japanese.stackexchange.com/q/76815/5010)




Your understanding seems fine.

  * **効能** [noun]: Refers to positive/expected effects of a therapy/medicine/onsen/etc.
  * **効き目** [noun]: Almost the same as 効能, but it is a wago (see [wago-and-kango](/questions/tagged/wago-and-kango "show questions tagged 'wago-and-kango'")) and tends to be avoided in academic contexts.
  * **効力** [noun]: Can be used in the same way as 効能 in therapeutic contexts, but mainly used in other contexts in the sense of validness/activeness, as in "the law is in _effect_ ", "the rule took _effect_ ".
  * **効果** [noun]: Almost the same as 効能 in therapeutic contexts. Also used in other contexts in the sense of illusion/tricks/decoration, as in "visual/audio _effect_ ".
  * **作用** [noun, suru-verb]: Tends to refer to direct effects at a more mechanical level (physics, physiology, etc). This is a neutral word that does not assume the effect is desirable. 副作用 ("side effect") is an important derivative word.
  * **影響** [noun, suru-verb]: Closer to "influence" rather than "effect".
  * **有効** [na-adj]: "Effective". The only na-adjective among these.

So "This drug is effective" can be translated as この薬には効能がある, この薬には効き目がある,
この薬には効果がある and この薬は有効だ. この薬には作用がある makes little sense unless this was said as
opposed to placebo.

Related:

  * [Difference between 影響 and 効果](https://japanese.stackexchange.com/q/76815/5010)

