Their blood smells... different. [Clearly a diplomatic word choice.] Werewolf blood ranges from dissatisfying to outright toxic, and my limited experience with other shapeshifters suggests it's a similar effect with other types.

[Normally he'd be leery exposing a weakness this way, and in truth, he isn't sure it's the right move. But she doesn't seem like the sort to lace his treatment just to see what happens. He hopes he's not misreading her.]