Before receiving communion, under penalty of serious sin, one must be fasting. We must respect the law of the Eucharistic fast before approaching the holy table. It consists of having taken no alcoholic drink or solid food, nor any liquid food, even non-alcoholic, for one hour. Water and medicines, even solid or alcoholic, do not break the fast at any time.
At present, ecclesiastical law has reduced the duration of the Eucharistic fast to just one hour, which is a loosening from the traditional discipline, which in its ancient and venerable form consisted in abstaining from all food or drink from midnight until morning communion.
The Society of St. Pius X encourages the faithful to adhere to the discipline in force before the Second Vatican Council, namely to observe a fast of three hours before the communion.
It would be committing the mortal sin of sacrilege to receive communion voluntarily without fasting, unless in danger of death or in the necessity of preventing the profanation of the sacrament.