Responses
It hasn't been revealed yet, but it's likely to be some kind of antidepressant or sleeping pills, meaning Dohu has been suffering depression and insomnia all this time. The part that is unclear is if Juheon realized that he was the cause. It feels that Juheon might have mistaken Gitae as the cause of Dohu's depression.

I don't understand the current situation, wts happening?? Can someone please explain?!.( ̄∇ ̄")