We should not consume so much alcohol that our will is diminished or we cannot function properly to the point that we could put ourselves or others in danger. To do so might cause us to do something we would not ordinarily do, and that something could involve grave matter.
This often happens on dates when a young woman or man drinks too much and then engages in sinful behavior that he or she would not have done otherwise.
Then there is the situation when someone drinks too much and then drives. They may become sleepy, their vision may become impared, they may have difficulty focusing on their driving, they may incorrectly judge speed or distance, or their reaction time may increase. Any of these factors could lead to an accident which could kill oneself or others.
We do not ever really know when someone commits a mortal sin. Three conditions must be met for mortal sin to occur.
1.There must be grave matter involved
2.There must be full knowledge
3.There must be full consent
If, through no fault of your own, number 2 or 3 condition is absent, guilt is diminished or possibly even absent. But if it is your fault that knowledge or consent is reduced or absent, then you could still be guilty of mortal sin, provided grave matter is involved.
