Objection 1. It would seem that Extreme Unction is not a sacrament. For just as oil is used on sick people, so is it on catechumens. But anointing of catechumens with oil is not a sacrament. Therefore neither is the Extreme Unction of the sick with oil.
Objection 2. Further, the sacraments of the Old Law were figures of the sacraments of the New Law. But there was no figure of Extreme Unction in the Old Law. Therefore it is not a sacrament of the New Law.
Objection 3. Further, according to Dionysius (Eccl. Hier. iii, v) every sacrament aims at either cleansing, or enlightening, or perfecting. Now Extreme Unction does not aim at either cleansing, or enlightening, for this is ascribed to Baptism alone, or perfecting, for according to Dionysius (Eccl. Hier. ii), this belongs to Confirmation and the Eucharist. Therefore Extreme Unction is not a sacrament.
On the contrary, The sacraments of the Church supply man's defects sufficiently with respect to every state of life. Now no other than Extreme Unction does this for those who are departing from this life. Therefore it is a sacrament.
Further, the sacraments are neither more nor less than spiritual remedies. Now Extreme Unction is a spiritual remedy, since it avails for the remission of sins, according to James 5:15. Therefore it is a sacrament.
I answer that, Among the visible operations of the Church, some are sacraments, as Baptism, some are sacramentals, as Exorcism. The difference between these is that a sacrament is an action of the Church that reaches to the principal effect intended in the administration of the sacraments, whereas a sacramental is an action which, though it does not reach to that effect, is nevertheless directed towards that principal action. Now the effect intended in the administration of the sacraments is the healing of the disease of sin: wherefore it is written (Isaiah 27:9): "This is all the fruit, that the sin... should be taken away." Since then Extreme Unction reaches to this effect, as is clear from the words of James, and is not ordained to any other sacrament as an accessory thereto, it is evident that Extreme Unction is not a sacramental but a sacrament.
Reply to Objection 1. The oil with which catechumens are anointed does not convey the remission of sins to them by its unction, for that belongs to Baptism. It does, however, dispose them to receive Baptism, as stated above (III, 71, 3). Hence that unction is not a sacrament as Extreme Unction is.
Reply to Objection 2. This sacrament prepares man for glory immediately, since it is given to those who are departing from this life. And as, under the Old Law, it was not yet time to enter into glory, because "the Law brought nobody [Vulgate: 'nothing'] to perfection" (Hebrews 7:19), so this sacrament had not to be foreshadowed therein by some corresponding sacrament, as by a figure of the same kind. Nevertheless it was somewhat foreshadowed remotely by all the healings related in the Old Testament.
Reply to Objection 3. Dionysius makes no mention of Extreme Unction, as neither of Penance, nor of Matrimony, because he had no intention to decide any question about the sacraments, save in so far as they serve to illustrate the orderly disposition of the ecclesiastical hierarchy, as regards the ministers, their actions, and the recipients. Nevertheless since Extreme Unction confers grace and remission of sins, there is no doubt that it possesses an enlightening and cleansing power, even as Baptism, though not so copious.
Source: Summa Theologica (New Advent)