It is clear that the majority of characters in this tragic play have problems with drugs and/or alcohol, but what is also clear is that because so many of the other characters struggle with their dependency on drugs and alcohol as well, it means they are unable to berate each other effectively because they recognise that they would be hypocrites to tell another person off for a fault that they have in their own lives. Note for example how Mary Tyrone changes her tune when she comes down to see Edmund having a drink. After turning to her husband and telling him off for not taking the dangers of drink seriously enough, a sudden "look of terror" comes into her eyes and she changes her tune:
But, of course, there's no comparison at all. I don't know why I--Forgive me for scolding you, James. One small drink won't hurt Edmund. It might be good for him, if it gives him an appetite.
There are two things going on here and both involve an unwillingness to face up to certain key truths that eventually threaten to rip this family apart. Firstly, Mary recognises that she is unable to lecture others about their dependency on alcohol when she is unable to escape her addiction to morphine. Secondly, to admit that it would be dangerous for Edmund to drink would be to admit that his life is in danger. It is much easier therefore to pretend and delude oneself that drinking alcohol is actually something normal to do, and may even be good for Edmund. Throughout the play therefore drugs and alcohol are two objects that expose the capacity of the characters to ignore the truth and delude themselves, putting off facing inevitable facts for as long as possible.