Monsterholic Vol. 1 (light novel)

Where beasts run wild, and monsters are real. Darkness hides behind the clean, safe, and orderly facade of society in Akitsushima. In the slum known as the Masquerade, people indulge in a drug called Monster Tonic, which transforms them into animal-human hybrids. There are no laws or restrictions in this government-designated area, and it's rife with immorality, obscenity, and murder. When bodies turn up in the streets, the police don't investigate, so it’s up to Reiji and Getsu to track down the killer. The two young men are just the pair for the job. Who better to hunt down a monstrous beast than a vampire and a werewolf?