How to say "ineffective" in Japanese

• 無効むこうJLPT N1approx.Common
  no-adjective, na-adjectiveCovers 'ineffective' in the formal sense of null and void: a contract, ticket, or vote with no legal or official force. Not the word for methods or efforts that simply fail to work.
  脅迫きょうはくのもとになされた約束やくそくは無効むこうだ。↗

  A promise given under a threat is worthless.
• 虚しいむなしいJLPT N1approx.Common
  i-adjectiveCovers 'ineffective' in the sense of futile or vain: efforts or resistance that come to nothing, often with a feeling of emptiness. This is the word for trying hard and achieving nothing.
  彼かれは多おおくの日々ひびをむなしく過すごさなければならなかった。↗

  He had to spend many barren days.

Use 無効 for 'ineffective' meaning formally invalid or void — lacking legal or official force — and 「虚しい」 for 'ineffective' meaning futile or vain, when efforts come to nothing and feel empty.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 無効むこうinvalid
• 虚しいむなしいvain