From the De Malo to the Summa Theologiae Aquinas apparently makes a shift in his judgment about drunkenness. While in the De Malo he says that getting drunk is of itself a venial sin, in the Summa Theologiae and in the Commentary on Romans (as well as the Commentary on 1 Corinthians), which are widely considered to be of a later date than the De Malo, he says that getting drunk is of itself a mortal sin.
In the De Malo, q.2, a. 8, Aquinas asks whether a circumstance can make a venial sin into a mortal sin. The third objection argues that getting drunk once is a venial sin, while getting drunk many times is a mortal sin. Hence a circumstance (the frequency) makes a venial sin into a mortal one. He replies:
To the third it should be said that getting drunk many times is not a circumstance that constitutes a species of sin, and therefore as getting drunk once is a venial sin, so getting drunk many times is a venial sin, speaking per se; but accidentally and by way of disposition getting drunk many times can be a mortal sin, as for example, when by the custom of drinking someone comes to have so great complacency in drunkenness that he would be willing to get drunk even if it involved the contempt of a divine precept. (De Malo, q.2, a. 8 )
Again in q. 7, a. 4, where the same question comes up again, a similar objection is raised:
1. Augustine says in a sermon on Purgatory that if anger is held onto for a long time, and if drunkenness is a regular (assidua) occurence, they are then numbered among mortal sins. But such sins are generically venial sins–otherwise they would always be mortal sins. Therefore a venial sin becomes mortal through the circumstance of regularity or duration. (De Malo, q. 7, a. 4)
He makes a similar reply, though with a different argument.
It should be said about drunkenness, that it in itself makes the reason actually not turned toward God, i.e., so long as the drunkenness lasts the reason cannot be turned toward God. And since a man is not obliged at all times to actually turn his reason towards God, drunkenness is not always a mortal sin; but when a man gets drunk regularly, it seems that he is not concerned about whether his reason is turned toward God, and in such a case drunkenness is a mortal sin, for it seems that on account of the pleasure of wine he despises the turning of his reason toward God. (De Malo, q. 7, a. 4, ad 1)
In I-II, q. 88, a. 5, he asks whether some circumstance of an act can make a venial sin into a mortal sin. The first objection is of particular interest, because it is almost exactly the same as that in De Malo, q. 7, a. 4. Aquinas writes:
Augustine says in a sermon on Purgatory that if anger is held onto for a long time, and if drunkenness is a regular (assidua) occurence, they are then numbered among mortal sins. But anger and drunkenness are not generically mortal sins, but venial sins–otherwise they would always be mortal sins. Therefore a circumstance makes a venial sin into a mortal sin. (I-II, q. 88, a. 5)
In reply, he says:
About drunkenness we should say that that it has in itself the character of a mortal sin; for when a man without necessity and merely for the sake of the pleasure in wine, make himself unable to use his reason, by which a man is directed to God and avoids committing many sins, such an act is expressly contrary to virtue. But it can be a venial sin on account of some sort of ignorance or weakness, as when a man is ignorant of the strength of the wine, or of his own incapacity (for drinking), so that he does not expect to get drunk; for in such a case the drunkenness is not imputed to him as a sin, but only the excessive drinking. If, however, he gets drunk frequently, this ignorance can no longer excuse him, and his will seems to choose drunkenness rather than refraining from an excess of wine; hence the sin becomes again what it is by its own nature [namely a mortal sin]. (I-II, q. 88, a. 5, ad 1)
Again in the Secunda Secundae, q. 150, a. 2, where he takes up drunkenness specifically, and asks whether it is a mortal sin, he gives the same reply:
Someone may be well aware that he is drinking immoderately and thereby getting drunk, and yet he would rather be drunk than abstain from drink. Such a man is the one who is properly speaking called a drunkard [in contrast to persons who drink too much without knowing it, or get drunk without expecting it], because moral character comes not from things that occur accidentally and aside from the intention, but from that which is directly intended. In this way drunkenness is a mortal sin, because then a man willingly and knowingly deprives himself of the use of reason, bu which he performs virtuous deeds and avoids sin, and thus he sins mortally by running the risk of committing sin. For Ambrose says in the book On the Patriarchs: "We say that one should avoid drunkenness, since it keeps us from avoiding grievous sins. For the things we avoid when sober, we unknowingly commit through drunkenness." Therefore drunkenness, speaking per se, is a mortal sin. (Secunda Secundae, q. 150, a. 2)
In the reply to the first objection he interprets Augustine's saying in the same way as he does in the Prima Secundae.
His treatment in his Commentary on Romans and on 1 Corinthians is very much like that in the Summa Theologiae.
How to Account for the Difference?
The divergence between the account in the De Malo and in the later writings could be explained in several ways:
(1) Aquinas may have become stricter in general, and thus stricter in his judgment of drunkenness. (This would be an interesting subject of research. I don't know of any studies investigating such a line of thought).
(1b) He may have gained more experience of the harmful things people do when drunk, and pronounces judgment accordingly.
(2) He may be envisioning quite different contexts. In the De Malo he may be envisioning, e.g., a monk in his cell who gets drunk by drinking too much wine, and the principal or only harmful consequence is that he can't pray or contemplate at that time, while in the other works he is envisioning a person engaged in various activities and in active relationships with other persons, who is liable to damage things or injure other persons if he is drunk.