Scarlet, Volume 1 (Scarlet #1-5)

Finé would do anything for her friend, Iris Redhood, even take the illegal magical drug Elixir and become a bloodthirsty monster. Iris would do anything for Finé, including letting Finé drink her blood, the only thing that can calm Finé down when the cravings get too much. But Iris's devotion comes at a price: she's actually a werewolf, and has plans to devour Finé whole!