I think it's either opium or something of a narcotic to relieve the pain from his wounds. If we take to consideration the wounds Shin receive from the last season, it's understandable how the author established him smoking something (considering there's no medicine for such pain in the story setting). Opium, except for being an addictive drug, can be an anaesthetic. The drawbacks on taking it is seen on the pannels where shin vomited, when he fell down due to either nausea or dizziness. (This is on a medical stand point hahaha) BUT storywise, the scenes on the chapters can be indicative of some other story line progression.
I think it's either opium or something of a narcotic to relieve the pain from his wounds. If we take to consideration the wounds Shin receive from the last season, it's understandable how the author established him smoking something (considering there's no medicine for such pain in the story setting). Opium, except for being an addictive drug, can be an anaesthetic. The drawbacks on taking it is seen on the pannels where shin vomited, when he fell down due to either nausea or dizziness. (This is on a medical stand point hahaha) BUT storywise, the scenes on the chapters can be indicative of some other story line progression.