When you use a drug / drink alcohol, you get a number of invisible addiction points. Every week, you lose some mood and one addiction point. You stop losing mood once all addiction points are gone.
So if you drink two beers in one week, then you will lose mood two weeks in a row.
Drugs / alcohol give you more mood than what addiction takes from you every week, so if you keep drinking every week, you will get more mood than what you lose. Doing this long term will give you a lot of addiction points and running out of drugs can poentionaly lead to game over, unless you find a way to offset the long term mood loss.