The height of medical science used to be applying leeches and administering useless poisons, like mercury. Undoubtedly many patients were killed by harmful treatments, and few if any were helped. Have historians of medicine estimated the year when calling a doctor first had a net positive impact on your chances of survival?
The problem with this question is that nobody can measure this "net impact", especially in the past. Very ancient (stone age) remains show that surgeries were performed, and not only on the limbs but also on the scull. Perhaps it helped in some cases and harmed in other cases, but how can one perform any statistics of this sort?
Even in the recent time, we know of course that average life expectancy increased dramatically, but how much of this can be attributed to medicine, and how much to the general improvement of life conditions (hygiene etc.)?
Nevertheless some milestones can be marked of course. Invention of smallpox vaccine in the late 18th century certainly had enormous effect. Also the discovery of germs as causes of many diseases in 19th century, antiseptic, and discovery of antibiotics in 20th century. There is no doubt that these discoveries saved many millions of lives.