Bishop Vitus Huonder of Chur's comments come after the number of euthanasia deaths in Switzerland rose by more than a third last year.
In a pastoral message to the country's priests, the Bishop said: "The readiness of a suffering patient to commit suicide with help from a bystander places any priest in an impossible situation if called to administer sacraments.
"Under such conditions, their reception is impossible - all a priest can do is offer a prayer of intercession and commend the dying to God's mercy."
He added that Church teaching was clear on euthanasia and that medical treatment should not "impair the natural process of dying."
"But from a Christian viewpoint, life and death are in God's hands - we do not decide about them for ourselves," he said, "suicide, like murder, contradicts the divine world order."
Euthanasia is legal in Switzerland and has dramatically increased in recent years, according to a report by local newspaper Neue Zürcher Zeitung.
The paper reported that 999 euthanasia deaths were recorded in 2015, up 35 per cent in just a year.