Scarlet Series by Chiri Yuino

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!

A LOVE THAT WILL DEVOUR THEM WHOLE Finé is under a terrible curse that causes her to transform into a bloodthirsty monster. Her werewolf lover Iris is determined to help Finé become human again. .. if only because Iris wishes to eat her. But when their enemies start moving against them, Finé loses control of the monster within. Can Iris bring her back from the brink? The thrilling conclusion to a supernatural yuri romance! The final volume!