Objection 1. It would seem that drunkenness does not excuse from sin. For the Philosopher says (Ethic. iii, 5) that "the drunkard deserves double punishment." Therefore drunkenness aggravates a sin instead of excusing from it.
Objection 2. Further, one sin does not excuse another, but increases it. Now drunkenness is a sin. Therefore it is not an excuse for sin.
Objection 3. Further, the Philosopher says (Ethic. vii, 3) that just as man's reason is tied by drunkenness, so is it by concupiscence. But concupiscence is not an excuse for sin: neither therefore is drunkenness.
On the contrary, According to Augustine (Contra Faust. xxii, 43), Lot was to be excused from incest on account of drunkenness.
I answer that, Two things are to be observed in drunkenness, as stated above (Article 1), namely the resulting defect and the preceding act. on the part of the resulting defect whereby the use of reason is fettered, drunkenness may be an excuse for sin, in so far as it causes an act to be involuntary through ignorance. But on the part of the preceding act, a distinction would seem necessary; because, if the drunkenness that results from that act be without sin, the subsequent sin is entirely excused from fault, as perhaps in the case of Lot. If, however, the preceding act was sinful, the person is not altogether excused from the subsequent sin, because the latter is rendered voluntary through the voluntariness of the preceding act, inasmuch as it was through doing something unlawful that he fell into the subsequent sin. Nevertheless, the resulting sin is diminished, even as the character of voluntariness is diminished. Wherefore Augustine says (Contra Faust. xxii, 44) that "Lot's guilt is to be measured, not by the incest, but by his drunkenness."
Reply to Objection 1. The Philosopher does not say that the drunkard deserves more severe punishment, but that he deserves double punishment for his twofold sin. Or we may reply that he is speaking in view of the law of a certain Pittacus, who, as stated in Polit. ii, 9, ordered "those guilty of assault while drunk to be more severely punished than if they had been sober, because they do wrong in more ways than one." On this, as Aristotle observes (Polit. ii, 9), "he seems to have considered the advantage," namely of the prevention of wrong, "rather than the leniency which one should have for drunkards," seeing that they are not in possession of their faculties.
Reply to Objection 2. Drunkenness may be an excuse for sin, not in the point of its being itself a sin, but in the point of the defect that results from it, as stated above.
Reply to Objection 3. Concupiscence does not altogether fetter the reason, as drunkenness does, unless perchance it be so vehement as to make a man insane. Yet the passion of concupiscence diminishes sin, because it is less grievous to sin through weakness than through malice.
Source: Summa Theologica (New Advent)