How to say "harmful" in Japanese

• 有害ゆうがいCommon
  na-adjective, nounCovers the core sense of 'harmful' — something that causes damage to health, safety, or the environment. It is the common, default word when you simply mean 'harmful' or 'hazardous'.
  煙草たばこは健康けんこうに有害ゆうがいである。↗

  Smoking is harmful to your health.
• 不為ふため
  na-adjective, nounA less common word meaning disadvantageous, harmful, or unprofitable. Its focus is on something working against one's interests rather than causing direct harm, so it is seldom the word you want for plain 'harmful'.

Use 有害 for the straightforward sense of something physically or health-wise harmful, and reserve 不為 mainly for the sense of being disadvantageous or unprofitable to someone.

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

• 有害ゆうがいharmful
• 不為ふためdisadvantageous