Yoshimasu Nangai 吉益南涯 (1750–1813), Japanese physician of late-Edo Kyōto, eldest son of 吉益東洞 Yoshimasu Tōdō and his successor as head of the kohō-ha 古方派 (Ancient-Formula School). Nangai softened his father’s rigid one-pathogen-one-poison (萬病惟一毒) doctrine into the qì-xuè-shuǐ 氣血水 (qì–blood–water) tripartite theory of pathology, allowing greater flexibility in clinical reasoning while preserving the Shānghán lùn formula canon. His principal exposition is Iji wakumon 醫事或問 / Yīhuò 醫或.