Depends on the arena of medicine to which you are referring. For the most part, everything outside of biochemistry was pretty darn solid.

From Hippocrates, to Aretaeus of Cappadocia, to Herophilus and Erasistratus, to Galen, they had near-complete mastery of anatomy, much of the brain's function had been mapped out (including cranial nerves), treatment of broken bones was fully mature, and the function of most organs had been figured out.

That being said, due to the lack of understanding of chemistry and microorganisms, there was also a lot of misinformation. The function of the liver, spleen, lymphatic system, tonsils, glands, etc. was a near-complete mystery. And where function was known, the mechanisms were misunderstood, particularly with the circulatory system, the kidneys, and the intestines.

Additionally, humoral pathology still reigned supreme (and persisted into the early 20th century). So no, antibiotics were not known per se. Pharmacy has been a thing for as long as there have been humans. The medicinal properties of plants and extracts can be observed, even if the reason behind their effectiveness is unclear. And there was no perception of a fully sterile environment to protect against microorganisms.

But all of this information persisted through the middle ages. None of it was lost, but it also wasn't really expanded upon much. Ironically, one of the big reasons was the ego of Galen, probably the most prolific (and also the best) of the ancient physicians. His corpus of contributions is massive. And groundbreaking. But he was also a dick. In his texts, he would berate and humiliate physicians who held different viewpoints from him. In doing so, he created an environment in which people were afraid to disagree with him, and that persisted. Going against Galen was grounds for derision. That ethos likely set back the advance of medicine by a few hundred years.

So yeah, all of early medical knowledge persisted through the middle ages. That's the only reason those texts survive today.