Would I be committing mortal sin if I refuse medical treatment?
It has long been my feeling that, if I become terminally ill, I would prefer to go to a hospice and let nature take its course rather than go through the agony of a few more years of a miserable life. Lately, though, I've begun to reconsider my position, as it seems to me that the refusal of medical treatment in this situation could be seen as suicide, albeit in a passive manner. What does the Church say about this? Would I be committing mortal sin if I refuse medical treatment?
In the situation you describe, the Church requires that ordinary means of maintaining life (nourishment and hydration, for example) not be denied. But it does allow the cessation of extraordinary means:
"Discontinuing medical procedures that are burdensome, dangerous, extraordinary, or disproportionate to the expected outcome can be legitimate; it is the refusal of ‘over-zealous’ treatment. Here one does not will to cause death; one’s inability to impede it is merely accepted. The decisions should be made by the patient if he is competent, whose reasonable will and legitimate interests must always be respected" (CCC 2278).