How to say "malignant" in Japanese

• 悪性あくせいCommon
  no-adjectiveThe medical sense: malignant cancer and tumors, virulent disease, pernicious anemia. This is the word you want when 'malignant' describes a growth or illness rather than a person's behavior.
  今こん悪性あくせいの風邪かぜがまん延えんしている。↗

  A bad cold is going about now.
• 悪質あくしつCommon
  na-adjective, nounThe figurative sense: malicious, vicious, underhanded. Use it when 'malignant' characterizes nasty intent or conduct, as in malicious pranks or underhanded schemes; for cancerous growths it is not the word — that is 悪性.
  悪質あくしつなドライバーは免許証めんきょしょうを取り上とりあげるべきだ。↗

  Bad drivers should have their licenses taken away from them.

Choose 悪性 for the medical sense — malignant cancer or tumors, virulent disease, pernicious anemia — and 悪質 for the figurative sense, where 'malignant' means malicious, vicious, or underhanded in intent or behavior.

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

• 悪性あくせいmalignant (cancer)
• 悪質あくしつmalicious
• たちが悪いたちがわるいof bad character
• たちの悪いたちのわるいof bad character