In the big city, drug crimes run rampant. Eto Kai and Kurabayashi Haru are two newcomer investigators who fight against them. In order to save people from modern society; poisoned with drugs, the two turn to face an invisible enemy...

From VIZ:

Kai Eto may look like a squeaky clean kid, but the Greater Kanto Narcotics Control Division's new investigator hides a violent alter ego and a dangerous past. With his stoic partner Hal Kurabayashi, Kai is assigned to track down and stop the distribution of Dragon Speed, a dangerous new drug. But Ryugen, the syndicate trafficking the drug, is almost impossible to infiltrate. Add street violence and a jurisdiction war with the Meguro Police, and poor Kai is in for some very long days at the office.

From Timcanpi:

"Switch" is a manga about the Matori, drug cops of the Narcotics Control Dept in Japan. Following a very unconventional plot, we are introduced to different situations the Matori must face throughout their career, through the eyes of a newbie criminal investigator and our main character, Eto Kai. However, as the manga progresses, we gradually begin to realize that Kai isn't all he appears to be. The series, by Naked Ape, is very edge-of-your-seat, and highly recommended for everyone.

The story centers around two newbies at the Narcotic Control Department in Japan, Hal and Kai. While struggling against drug dealers, gangs, murderers, psychopaths, mysterious organizations and mad people in general, they have also to deal with their own mysteries, specially Kai. The gentle and caring boy, who suffers together with the drug users and tries his best to stop people from suffering, just turns into a killing machine suddenly in some cases, like the occurrence of a certain event functioned like a switch. What's behind this? Who is he, in fact?