It’s common to hear about such medical practices as blood letting and using mercury for illness. Is there a general time period where doctors started to overall do more good than harm?

There were also remedies that worked and were known to work thanks to observational skills. While it is true that a doctor of 2021 is going to give you better medical care than a doctor of 921, it's not like you're instantly dead if you go to the doctor of 921, especially if they're using Bald's Leechbook. I bring you a compilation of medical posts as previously seen on my Middle Ages Is Best Ages Compilation.

u/BedsideRounds has a non-exhaustive list of Stuff That Worked;
u/sunagainstgold talks about medicine and its efficacy, as well as medical education in the Late Middle Ages;

But the star of this post is u/BRIStoneman drawing from the 9th Century medical text Bald's Leechbook:

On the book's attitude to the treatment of medical conditions in general;
On the treatment of infected wounds;
On its recommendations for cuts and fevers (with bonus literacy downthread);
An assortment of treatments demonstrating that the leeches of Early Medieval England were on to something;
On its treatments for worms of all kinds.

Bonus: Bald's Leechbook is available in all its digitised glory here.

As ever, more insight is welcome from anyone with the knowledge, so if you'd like to type up a post of your own, please don't let this post stop you!