Born in 1760 into a medical family at Hirayama in Naga district, Kii province (present-day Kinokawa, Wakayama Prefecture). Having studied both Kohōha (Ancient Prescription) and Western medicine in Kyoto, he set up practice in his hometown. Inspired by Hua Tuo's description of mafeisan, he devoted his life to research on a general anesthetic. He compounded six herbs including datura (Korean morning glory / Datura metel) and aconite, conducting repeated animal experiments. His mother Otsugi and his wife Kae volunteered as human test subjects — Kae lost her sight to the side effects. On October 13, 1804, he successfully performed a mastectomy on the 60-year-old breast cancer patient Aiya Kan under general anesthesia induced by his formula, tsūsensan. This was 42 years before ether anesthesia in the West (1846) and the world's first surgery under general anesthesia. Over the next thirty years he performed over 150 breast cancer operations and trained more than a thousand disciples at his Shunrin-ken school. He died in 1835.

An indomitable seeker. He pursued a single goal — general anesthesia — through more than twenty years of research, with a tenacity that never wavered. At the same time, he was a man of such character and conviction about medicine that he could ask his own family to bear the cost of the experiments and have them accept it. Seeking no fame, he polished his medical art in silence in Wakayama during the age of national isolation.

Seishū's 1804 operation is still highly esteemed as a luminous achievement in world medical history. The American Society of Anesthesiologists bears his name on its logo, and his achievement is internationally recognized. In Kinokawa City, the 'Seishū no Sato' memorial site preserves and reconstructs the Shunrin-ken academy buildings. Ariyoshi Sawako's novel The Doctor's Wife (1966) depicts the tension between mother and wife, making Seishū a household name. He is a figure inscribed in Japanese cultural history through both medicine and the family drama.