Hiya, I'm not sure I am the best to provide an overall picture of the profession in the 1800s (it would depend on the exact era and region), and I don't have much time to write a proper answer today, sadly, but I have written some answers that might give you some insight:

Here I write about patent medicines, which definitely fall under what you might call "snake oils" and hard drugs.

Here I wrote about opium, medicinal and other uses.

And here you can read about the medicinal use of arsenic in the 19th century.