מתן תרופה שעלולה לקרב את המוות

This is an urgent request for guidance. My grandfather has lung cancer and has very severe difficulty breathing. His cancer is no longer responding to chemotherapy or radiotherapy and we have been told that he has only a short time to live. He is suffering terribly, mainly because he cannot breathe. His doctors have suggested that he be started on a morphine infusion. They said that this may relieve his suffering, but that it also may make him drowsy and that it may slow his breathing. Although I am satisfied that this is not euthanasia, I am concerned that slowing his breathing may shorten the amount of time he has to live. I know that everything must be done to help him live longer and I am concerned that giving morphine is an ‘aveira. I have asked my family not to give consent until I have spoken to a credible halakhic authority and he says it’s permissible.