Paracelsus

Burned Galen in public, founded chemistry-as-medicine, and prescribed mercury for everything.

Paracelsus (Theophrastus von Hohenheim) declared war on the Greco-Roman medical canon — burning Galen and Avicenna in a bonfire at Basel — and replaced it with iatrochemistry: the idea that disease has chemical causes and chemical cures. He introduced zinc to European nomenclature, used mercury compounds to treat syphilis (effective, often fatal), and pioneered dose-dependent thinking ("the dose makes the poison"). He also taught a doctrine of signatures (plants shaped like organs heal those organs) and a cosmology of three primes — mostly wrong. He moved medicine from herbs-as-tradition to chemistry-as-mechanism. The mechanism part stuck.