Durarara!! dj - Start in

Welcome to Ikebukuro, where Tokyo's wildest characters gather!! Meet an ordinary boy who daydreams about the extraordinary. A naive stalker girl. The strongest man in Ikebukuro. A shut-in doctor with questionable credentials. A hedonistic informant...and the "headless rider" astride a pitch-black motorcycle!? As their paths cross, this eccentric cast weaves a twisted, cracked love story... (Source: Yen Press)

Pairing: Shizuo x Izaya Isaya ask Namie for a drug to kill shizuo and that she is the one to drug him in exchange of things about his brother. But the drugs acts anormally for the body nature of Shizuo... and the drug seem to be working even after. What is happening here?