The Apothecary Diaries 1 (Japanese Edition) by Natsu Hinata

Maomao, a former pharmacist in the red-light district, is currently serving as a servant in the Emporers Palace. The girl, who is by no means beautiful, is waiting for her contract to end hopefully without incidents. She soon learns that the emperor's children all have short life spans and upon hearing that the two surviving children are also seriously ill, Maomao begins to investigate the cause. This highly acclaimed, thrilling mystery is now available in a long-awaited paperback edition. Set in medieval Eastern Asia, a young "poison taster" solves one difficult case after another that occurs in the palace.