Mahou Shoujo to Mayaku Sensou

"Candy," a mysterious drug whose main ingredient is just ordinary sugar, is spreading like wildfire in Tokyo. When Susui, a narcotics agent who has infiltrated the yakuza to get closer to Candy's manufacturer, finds himself in danger, a self-proclaimed "magical girl" named Riri Hoshina comes to his rescue...!

(Source: Manga Plus)