Poison Study (The Chronicles of Ixia 1) by Maria V Snyder

Choose: A quick death or a slow poison
Locked deep in the palace dungeon for killing her abuser, Yelena knows she'll never be free again. The laws in Ixia are strict, and murderers must be executed. But just as she's resigned herself to her fate, Yelena is offered an extraordinary reprieve.

As the food taster, she will eat the best meals and have rooms in the palace — but risk assassination by anyone trying to kill the Commander of Ixia. To make matters worse, the chief of security deliberately feeds her a deadly substance, and only by appearing for her daily antidote will she delay an agonizing death from the poison.

As Yelena comes to terms with her new position, rebels plot to seize Ixia, and her life hangs in the balance. To survive, she must unravel the secrets behind the past she's been running from, and the mysterious powers that are suddenly coming to the surface..