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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.

1 Answer 1

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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 ) 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.

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