Objection 1. It would seem that drunkenness is not a mortal sin. For Augustine says in a sermon on Purgatory [Serm. civ in the Appendix to St. Augustine's works] that "drunkenness if indulged in assiduously, is a mortal sin." Now assiduity denotes a circumstance which does not change the species of a sin; so that it cannot aggravate a sin infinitely, and make a mortal sin of a venial sin, as shown above (I-II, 88, 5). Therefore if drunkenness is not a mortal sin for some other reason, neither is it for this.
Objection 2. Further, Augustine says [Serm. civ in the Appendix to St. Augustine's works]: "Whenever a man takes more meat and drink than is necessary, he should know that this is one of the lesser sins." Now the lesser sins are called venial. Therefore drunkenness, which is caused by immoderate drink, is a venial sin.
Objection 3. Further, no mortal sin should be committed on the score of medicine. Now some drink too much at the advice of the physician, that they may be purged by vomiting; and from this excessive drink drunkenness ensues. Therefore drunkenness is not a mortal sin.
On the contrary, We read in the Canons of the Apostles (Can. xli, xlii): "A bishop, priest or deacon who is given to drunkenness or gambling, or incites others thereto, must either cease or be deposed; a subdeacon, reader or precentor who does these things must either give them up or be excommunicated; the same applies to the laity." Now such punishments are not inflicted save for mortal sins. Therefore drunkenness is a mortal sin.
I answer that, The sin of drunkenness, as stated in the foregoing Article, consists in the immoderate use and concupiscence of wine. Now this may happen to a man in three ways. First, so that he knows not the drink to be immoderate and intoxicating: and then drunkenness may be without sin, as stated above (Article 1). Secondly, so that he perceives the drink to be immoderate, but without knowing it to be intoxicating, and then drunkenness may involve a venial sin. Thirdly, it may happen that a man is well aware that the drink is immoderate and intoxicating, and yet he would rather be drunk than abstain from drink. Such a man is a drunkard properly speaking, because morals take their species not from things that occur accidentally and beside the intention, but from that which is directly intended. On this way drunkenness is a mortal sin, because then a man willingly and knowingly deprives himself of the use of reason, whereby he performs virtuous deeds and avoids sin, and thus he sins mortally by running the risk of falling into sin. For Ambrose says (De Patriarch. [De Abraham i.]): "We learn that we should shun drunkenness, which prevents us from avoiding grievous sins. For the things we avoid when sober, we unknowingly commit through drunkenness." Therefore drunkenness, properly speaking, is a mortal sin.
Reply to Objection 1. Assiduity makes drunkenness a mortal sin, not on account of the mere repetition of the act, but because it is impossible for a man to become drunk assiduously, without exposing himself to drunkenness knowingly and willingly, since he has many times experienced the strength of wine and his own liability to drunkenness.
Reply to Objection 2. To take more meat or drink than is necessary belongs to the vice of gluttony, which is not always a mortal sin: but knowingly to take too much drink to the point of being drunk, is a mortal sin. Hence Augustine says (Confess. x, 31): "Drunkenness is far from me: Thou wilt have mercy, that it come not near me. But full feeding sometimes hath crept upon Thy servant."
Reply to Objection 3. As stated above (Question 141, Article 6), meat and drink should be moderate in accordance with the demands of the body's health. Wherefore, just as it happens sometimes that the meat and drink which are moderate for a healthy man are immoderate for a sick man, so too it may happen conversely, that what is excessive for a healthy man is moderate for one that is ailing. On this way when a man eats or drinks much at the physician's advice in order to provoke vomiting, he is not to be deemed to have taken excessive meat or drink. There is, however, no need for intoxicating drink in order to procure vomiting, since this is caused by drinking lukewarm water: wherefore this is no sufficient cause for excusing a man from drunkenness.
Source: Summa Theologica (New Advent)