How does the story go, “hey critical patient, yo

How does the story go, “hey critical patient, you are very likely dieing, we have this drug that has a significant chance of saving you, and if it does not it is not because of any side effects of the the drug, so we are not going to try it” is this even possible? I worry that the # of critical patient is a problem, but how can the fda even think about not approving a drug that any healthy person could accidentally inject and have no side effects? What is the downside to at least give doctors the tool to try?