Objection 1. It seems to be lawful for a priest to refrain entirely from consecrating the Eucharist. Because, as it is the priest's office to consecrate the Eucharist, so it is likewise to baptize and administer the other sacraments. But the priest is not bound to act as a minister of the other sacraments, unless he has undertaken the care of souls. Therefore, it seems that likewise he is not bound to consecrate the Eucharist except he be charged with the care of souls.
Objection 2. Further, no one is bound to do what is unlawful for him to do; otherwise he would be in two minds. But it is not lawful for the priest who is in a state of sin, or excommunicate, to consecrate the Eucharist, as was said above (Article 7). Therefore it seems that such men are not bound to celebrate, and so neither are the others; otherwise they would be gainers by their fault.
Objection 3. Further, the priestly dignity is not lost by subsequent weakness: because Pope Gelasius I says (cf. Decretal, Dist. 55): "As the canonical precepts do not permit them who are feeble in body to approach the priesthood, so if anyone be disabled when once in that state, he cannot lose that he received at the time he was well." But it sometimes happens that those who are already ordained as priests incur defects whereby they are hindered from celebrating, such as leprosy or epilepsy, or the like. Consequently, it does not appear that priests are bound to celebrate.
On the contrary, Ambrose says in one of his Orations (xxxiii): "It is a grave matter if we do not approach Thy altar with clean heart and pure hands; but it is graver still if while shunning sins we also fail to offer our sacrifice."
I answer that, Some have said that a priest may lawfully refrain altogether from consecrating, except he be bound to do so, and to give the sacraments to the people, by reason of his being entrusted with the care of souls.
But this is said quite unreasonably, because everyone is bound to use the grace entrusted to him, when opportunity serves, according to 2 Corinthians 6:1: "We exhort you that you receive not the grace of God in vain." But the opportunity of offering sacrifice is considered not merely in relation to the faithful of Christ to whom the sacraments must be administered, but chiefly with regard to God to Whom the sacrifice of this sacrament is offered by consecrating. Hence, it is not lawful for the priest, even though he has not the care of souls, to refrain altogether from celebrating; and he seems to be bound to celebrate at least on the chief festivals, and especially on those days on which the faithful usually communicate. And hence it is that (2 Maccabees 4:14) it is said against some priests that they "were not now occupied about the offices of the altar... despising the temple and neglecting the sacrifices."
Reply to Objection 1. The other sacraments are accomplished in being used by the faithful, and therefore he alone is bound to administer them who has undertaken the care of souls. But this sacrament is performed in the consecration of the Eucharist, whereby a sacrifice is offered to God, to which the priest is bound from the order he has received.
Reply to Objection 2. The sinful priest, if deprived by the Church's sentence from exercising his order, simply or for a time, is rendered incapable of offering sacrifice; consequently, the obligation lapses. But if not deprived of the power of celebrating, the obligation is not removed; nor is he in two minds, because he can repent of his sin and then celebrate.
Reply to Objection 3. Weakness or sickness contracted by a priest after his ordination does not deprive him of his orders; but hinders him from exercising them, as to the consecration of the Eucharist: sometimes by making it impossible to exercise them, as, for example, if he lose his sight, or his fingers, or the use of speech; and sometimes on account of danger, as in the case of one suffering from epilepsy, or indeed any disease of the mind; and sometimes, on account of loathsomeness, as is evident in the case of a leper, who ought not to celebrate in public: he can, however, say mass privately, unless the leprosy has gone so far that it has rendered him incapable owing to the wasting away of his limbs.
Source: Summa Theologica (New Advent)