Shin'eiyaku

are pills designed by Kisuke Urahara to help Shinigami who have lost their Bankai to Quincy Medallions.

Overview
By temporarily causing the user of a Bankai to Hollowfy, it effectively makes the Bankai "poison" for a Quincy due to their inability to take in Hollow Reiatsu without their souls being destroyed in the process. The pills are taken through any means of physical contact. The effects were first seen when Tōshirō Hitsugaya's stolen Bankai, Daiguren Hyōrinmaru, broke and suddenly injured Cang Du. The Bankai then gradually returns to the Shinigami owner, with an additional effect of the Shinigami being Hollowfied, such as Hitsugaya briefly gaining a mask of ice over his left eye.