Heads - Manga

This manga's story is based on a thriller by Keigo Higashino. The main protagonist, the shy Jun Naruse, is a rather average guy who works for a electronic company and loves to draw. He falls in love with Megumi whom he met at a shop selling drawing materials and an intimate relationship develops. A few days later the office gets attacked and when the criminal tries to shoot a little girl he saves her by jumping in front of her. A month passes until he wakes up again and is told that half his brain got destroyed and he only survived because someone donated half a brain. Everything seems to be fine, he quickly recovers and is happy, however he begins to change...is the donated brain the reason for that? Or is it just a side effect of the medicine?