Apothecarius Argentum by Tomomi Yamashita (amazon.com)

Apothecarius Argentum deals with the relationship between a princess and her former poison tester. Primula is the type of princess who prefers sword fighting to etiquette lessons. As a boy, Argent tasted every morsel of food before Primula was allowed to eat. He’s a basilisk, someone exposed to many poisons at a young age which causes him to build up a tolerance to toxic substances. As a side effect, his touch can kill. Basilisks are often used as assassins as well as food tasters.

Years later Argent has become an apothecary and returns to the kingdom of Beazol. He’s summoned to the castle when Primula becomes sick, and she recognizes him as her childhood companion. They start to connect again as friends, but someone seems to be trying to poison the princess. Argent resumes his role as a protector, but his poisonous touch and the difference in their stations might prevent their friendship from developing.

This three volume series would be a good fit for most YA manga collections – it wouldn’t take up too much shelf space, and it has a bunch of read alike potential for fans of fantasy stories with untraditional princesses.
Dr. Scott is doing a medical annotation of the series, so you can learn all about the poisons mentioned in the manga.