Koukyuu no Dokuka

A "poison queen" who knows all kinds of poisons and a boy who is a "cross-dressing queen" solve the mysteries that lurk in the palace!

The time is the Great Tang Dynasty. The boy Yu Lan enters the palace as a disguised as a woman in place of his sister who has disappeared. He meets Dudu, a poison queen who claims to be able to use all kinds of poisons, both ancient and modern. The two are challenged to solve the mysteries of the mysterious incidents occurring in the imperial palace.

(Source: Kadokawa, translated)