Can someone's quality of life ever be so bad that you are justified in taking care of them against their will in order to improve it? If so, how bad does it have to be?

If I own something that is essential for other people to live, like medicines, and I know that I have made it impossible for them to afford it, am I responsible for their death?

When is it time to get on the medical intervention train and when should you leave well enough alone? As I have gotten older, and my friends have gotten older, every doctor visit feels like stepping on a train that will soon speed up so much you can't get off - so fast you can't even see the landscape outside the window anymore. Chronic medication, more tests...preventive examinations, just one more, just one more. I understand that these things prolong life, but how do you distinguish treatable medical conditions from normal aging? How do you go gently into that good night?

Suppose that I'm working on a medical treatment for a project with no known cure or even treatment. My subjects report that they feel much better after receiving the treatment, but subsequent study shows that the treatment is, in fact, ineffective and all that I'm seeing is the placebo effect. Can I ethically tell them the truth and thereby make them feel worse subjectively? Would that violate the "do no harm" principle of medical ethics?