Primitive medicine was any medical treatment that was seen as outdated and therefore less effective.
Compared to late 23rd century medicine, archaic treatments such as image therapy and chemotherapy were considered akin to the (medicine of the) Dark Ages. (Star Trek IV: The Voyage Home)
In 2370, Benjamin Sisko criticized Alixus for relying on primitive medicine to treat a dying Meg when more advanced care was just a call away. (DS9: "Paradise")