Paracelsus accepted Galen’s writing and wanted to prove that it was valid, this started a process that eventually showed that Galen was wrong.
There was no clear distinction between religion, science, and tribal or cultural myths.
A respected healer, could on the whim of a noble or the anger of a crowd be burned as a witch.
Natural cures, usually herbs; like foods, were generally considered “safe.”
Chemicals as a Food, Medicine or Poison
A doctors main skill might have been to read the Bible. He might also have traveled to Italy, or Turkey or Egypt. Muslim medicine was more scientific, but little known in Western Europe.
Most doctors refused to mix medicines. Probably a wise move, so the person who did mix the medicine could always be blamed if things went wrong.