Dokuonna, Ayamatte Outaishi wo Otosu

"I am no longer fit to marry the Crown Prince!" After being poisoned repeatedly by her stepmother, the eccentric noblewoman Belladonna developed a resistance to poisons - so much so that she eventually grew to enjoy eating toxic plants. Through relentless effort, she achieved her dream of becoming a botanist. Just as her career begins to flourish, an unexpected marriage proposal arrives - from none other than the Crown Prince himself! But life as a crown princess would mean abandoning the research she loves. Desperate to escape the engagement, Belladonna hatches a plan: she asks her close friend from the tavern, Will, to help her lose her virginity, hoping to render herself unfit for marriage. On the day of the prince's visit, she prepares to announce that she's no longer a maiden and expects the engagement to be called off. However, the man who appears before her is...!? (Source: MangaPlaza)