"Catholics who have received their First Sacraments, who are not conscious of grave sin, and who have fasted for at least one hour are encouraged to devoutly and frequently receive Holy Communion.
This is very important:
To respond to this invitation we must prepare ourselves for so great and so holy a moment. St. Paul urges us to examine our conscience: "Whoever, therefore, eats the bread or drinks the cup of the Lord in an unworthy manner will be guilty of profaning the body and blood of the Lord. Let a man examine himself, and so eat of the bread and drink of the cup. For any one who eats and drinks without discerning the body eats and drinks judgment upon himself." Anyone conscious of a grave sin must receive the sacrament of Reconciliation before coming to communion. (CCC 1385)
Translation: taking this Sacrament with serious, unconfessed sin on the soul brings condemnation rather than salvation!
We are also required to fast for just one hour before receiving. Water and medicine do not break a fast. Catholics are obliged to receive this sacrament at least once per year, if possible during Easter (CCC 1388).
Non-Catholics are not ordinarily admitted to Holy Communion and are asked to pray that the action of the Holy Spirit will draw us closer together and begin to dispel the sad divisions which separate us (see CCC 1398-1401)."
- taken from Bayou Catholics, the Web site of St. Elizabeth of Hungary and St. Jules parishes in the Catholic Diocese of Baton Rouge
Because Catholics believe that the celebration of the Eucharist is a sign of the reality of the oneness of faith, life, and worship, members of those churches with whom we are not yet fully united are ordinarily not admitted to Holy Communion. Eucharistic sharing in exceptional circumstances by other Christians requires permission according to the directives of the diocesan bishop and the provisions of canon law (canon 844 § 4). Members of the Orthodox Churches, the Assyrian Church of the East, and the Polish National Catholic Church are urged to respect the discipline of their own Churches. According to Roman Catholic discipline, the Code of Canon Law does not object to the reception of communion by Christians of these Churches (canon 844 § 3).
For those not receiving Holy Communion
All who are not receiving Holy Communion are encouraged to express in their hearts a prayerful desire for unity with the Lord Jesus and with one another.
We also welcome to this celebration those who do not share our faith in Jesus Christ. While we cannot admit them to Holy Communion, we ask them to offer their prayers for the peace and the unity of the human family."
- from the USCCB directive on "Non-Catholics and Holy Communion"