Bitten (2008)

"Relationships can be draining."

A lovelorn paramedic rescues a woman left for dead after a deadly assault. When she refuses to go to the hospital, he takes her in, nurses her back to health and soon falls in love with her. Before long the mysterious woman suffers withdrawal symptoms, leaving the paramedic to believe his newfound love is a drug addict. Coming home to find a blood-drained corpse on his floor, he learns she does indeed have an addiction problem but it's not to drugs...