Dokuhime

It starts with poisonous herbs under the cradle. Then under the sheets. And inside the clothes. Even mixed in the milk that they feed the newborn. This way the child gradually gets used to poisons and becomes the perfect assassination tool—the "Poison Princess" whose every kiss, tear, and even touch bring death. Her only chance of survival is to fulfill her duty as an assassin and find a way to live on in the enemy land she is sent to...if her heart is strong enough.

(Source: Aerandria Scanlations)