The first hurdle, it will require two doctors and a high court judge to give the go ahead so not completely within your own control. These doctors will be giving someone the green light to die, there will be cases where they may not be confident enough or without some doubt to say this person will be dead within six months so will they defer the decision or just think we are confident they are terminal so dead tommorow or at some point in the near future makes no difference as they are dead either way. These doctors will have to live with these decisions which go against there very basic desire of saving and promoting life.
That is the worry if you think about politics and decision making where what is said one day becomes something very different at another time so will we get to the point where we decide that poor old granny is dying and has no dignity so lets just wheel her down to the undertakers and put her down. Maybe we could end up with the full package, dial a death so the doctors watch as you take the pills and then your body is placed into the mobile incinerator where following a puff of smoke your ashes are dispensed into a container and given back to you and they move onto the next patient.
I think allowing someone to make that decision is there decision to make but there is a difference in being in pain and being terminal so the medical evidence needs to be very sound to prevent someone in pain and curable from making the wrong decision. Is this not why we got rid of the death penalty, because mistakes were made and people hung that were later found to be innocent, oh dear to late.