効力|こうりょくkouryoku[1]atamadaka (head-high)

1. nouneffect; efficacy; validity; potency
   その法律ほうりつはもう効力こうりょくはなくなっている。↗

   The law is not in effect any longer.

• Synonym効き目ききめNeutralHow strongly a medicine or the like takes effect; common in speech. 「効力」 is more formal and also covers the validity of laws and contracts.
• Synonym効能こうのうWrittenWritten word for the beneficial effects of medicines or hot springs, as in 「効能書き」. 「効力」 stresses the working force or whether something is valid.
• Synonym効果こうかNeutralThe most widely used word for an effect or action. 「効力」 emphasizes the power to work and whether something is valid.
• Synonym実効じっこうWrittenWritten word for actual, practical effect, as in 「実効性」. 「効力」 focuses on whether something has validity.
• Synonym効こうWrittenWritten word for efficacy; seldom used alone, appearing mostly in set phrases such as 「効を奏する」.
• Synonym利きききCasualColloquial word for how a drug takes effect, as in 「薬の利きが悪い」. 「効力」 is the more formal choice.
• Synonym効験こうけんWrittenWritten word for efficacy, with religious or medicinal overtones.
• Antonym無効むこうWrittenWritten word for having no validity, as in 「契約が無効になる」; the direct opposite of 「効力」.

Use 「効力」 for the force that makes a law or contract valid or a drug work, as in 「効力を失う」「薬の効力」. For effects in general say 「効果」; for a medicine's effects 「効き目」 or 「効能」; for practical effect 「実効」; for no validity 「無効」.

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

1. 物事ものごとが作用さようして生しょうじる、効き目ききめや力ちから。
2. 法律ほうりつやとりきめなどがもつ有効性ゆうこうせい。