Switch

Switch by Naked Ape was one manga that really had me excited. I couldn't get enough of it! It's genres are action, comedy, drama, mature, mystery, and psychological. Switch was another manga I started reading years ago and was removed from the manga site :'(  I actually just found it again online and the site will be listed below.

Summary:
(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.

This manga was made into a short anime consisting of two OVAs, I believe you can watch these on Youtube.com.

You can read this on Mangareader.net but I recommend buying it for you collection ;)

I have seen all thirteen volumes on Amazon.com, but also in bookstores. 

Picture found on Google*