What is the ruling on naming medicine "healing" or saying that the doctor heals, and is it true that the Messenger of Allah, may Allah bless him and grant him peace, said: "Indeed, excessive laughter deadens the heart"? And is there a relationship between the word "deadens" and the issue of "heals" by way of analogy?

Naming medicine as "healing" (شفاء) or saying "the doctor heals the patient" is a figurative intellectual attribution (مجاز عقلي), where the action is ascribed to other than its true doer, for Allah Almighty is the true Doer of these matters. As for the hadith, "Indeed, much laughter deadens the heart," it is authentic.