Yonaoshi Jyunan! Ninjouken

Junan, a former instructor of swordplay for the Shogun family, went to Nagasaki and studied medicine. After coming back to Edo, he becomes a highly skilled doctor that he is now also a medical examiner for the magistrate's office. But no one knows of his the other side: he goes out and punishes the evil using his secret weapon… “needle of death.”
(Source: Wiki.d-addicts)