One of the strangest problems in Long Day's Journey into Night is that the fundamental conflict of the play relies on an inherent hypocrisy. The issue at the center of the play is Mary's addiction to morphine and the attempts of the Tyrone men to prevent it. But James, Jamie, and Edmund—all hell-bent on treating Mary's addiction and keeping her from relapsing—each deal with addiction themselves (to alcohol). This creates a sense of situational irony that affects the entire play, as the addicts try in vain to help the addict.
Everyone in the Tyrone family drinks, and each thinks that everyone else ought to drink less. The most prominent instance of this is between James and Jamie. From the beginning of Act One, James chastises his son for drinking, saying: "You've thrown away your salary every week on whores and whiskey!" In Act Two, Edmund sneaks extra whiskey with Cathleen while no one's home. Cathleen, "suddenly primly virtuous," knows that this is bad: "I'd never suggest a man or woman touch drink, Mister Edmund." But, "relenting," she says that a "drop now and then is no harm when you're in low spirits." By Act Four, all the men are all deeply drunk, James having drunk three-quarters of a bottle of whiskey by himself. Even still, James doesn't want Edmund to drink anymore: "It's too much in your condition," referring to his tuberculosis. All the characters know drinking is bad, but they see it as "no harm when you're in low spirits." Alcohol, for the Tyrones, is a necessary and excusable evil.
It is odd, then, that the Tyrone men extend none of this sort of forgiveness to Mary for her morphine addiction and her various relapses. James shouts at her for using "God-damned poison." The men spy on Mary to see if she might be using drug, but no such espionage occurs in the house over alcohol. The closest thing is Cathleen's assertion that James knows how full the whiskey bottle is, but anyone in the house can easily fool him by watering it down to fill the bottle back up after they take a drink. James is the heaviest drinker in the family and he, in particular, is most adamant that Mary shouldn't use morphine. Mary's drug use and addiction receives no good-natured leeway like the alcoholism of the men.
Thus, the play has a consistent and all-encompassing irony: alcohol and morphine are both vices to help the family's pain and suffering, but one is forgivable whereas one is not. Why this is the case is difficult to answer: morphine is indeed especially addictive and harmful, more so than alcohol. But the alcoholism in the family is so extreme, especially in the case of James, that the difference between them may be negligible. It is also worthy of note that morphine addiction was common among upper-class women in the United States in the early 20th century. Perhaps the family's insistence on preventing Mary's addiction is due to a combination of her gender, her particular addiction, and her status as caretaker in the family. The answer is not made clear in the play. In any case, this irony continues through the entire work, adding a sense of injustice to the overriding tension and fear.