I think it depends how you frame this question. For example, aspirin originated from a compound in willow bark. Currently, most of us aren’t chewing on willow bark to relieve pain, but at some point that (or making willow bark teas, etc.) was the case. Whether you consider that as “alternative” depends on what point in history you ask that question.

Leeches, which are still used today, also come to mind. Modern medicine has greatly refined how they’re used, but that’s not how it started.

There are quite a few things that people noticed produced effects but didn’t understand the mechanism. Science changed that.

However, if you consider using things with known properties in a wildly irresponsible manner as the definition of “alternative medicine,” that’s a very different question.

Even when novel therapies from existing medicine do come to light at times, generally this is through scientific observation and testing, not Cletus taking horse medicine.