I have a serious squick for anything that is related to anyone taking drugs willingly - a big, serious squick that has made me avoid fics that people have assured me are quite excellent. Let's say I had a friend in RL, and personal traumas and all that.
And yours, I must admit, was the first one I have read that included this.
And I don't regret it, strangely enough. You made them so perfectly characterized - they are both bitter, lonely men finding comfort in each other. And Filch's obviously adoration of Snape. They are both prickly, and callous, and so utterly human.