The label often says it all. “For Medicinal Purposes Only.” It’s a strange, formal phrase to find on a bottle of whiskey, a relic from a time when the American government simultaneously banned alcohol and licensed a select few to produce it under doctor’s orders.

These bottles are now the focus of a specific kind of collector. They are hunting for liquid history, for a spirit that exists outside the modern world. The story begins in 1920 with the 18th Amendment. While Prohibition shut down the country’s distilleries, a neat loophole allowed six producers to continue their work. Names like Brown-Forman and the company that would become Stitzel-Weller were granted licenses to distill whiskey for medicinal use, prescribed by a doctor and filled at a local pharmacy.

Read more about collecting here.