“Bitters seemed to have originated in the United States being used mostly for flavoring and tonic purposes. They are used extensively on the continent, especially so in France, where they are regarded favorably as a substitute for Absinthe. The excessive use of bitters as a tonic should be avoided as they have a tendency to weaken the stomach.” — John L. Caulkins, The Mixing Master, 1933
Bitters were originally developed as “tonics” — aka patent medicines. They were advertised as miracle cure-alls, and were usually (seemingly) effective, due to the fact that they contained such wonders as opium, cocaine, and of course alcohol.
Coca Cola is probably the most famous tonic. Early marketing materials highlight its ability to banish “mental and physical exhaustion”. This wasn’t false advertising; the drink’s original recipe included koca nuts (caffeine) and coca leaves (from which with a bit of labor, you get cocaine).
The Mixing Master manuscript has several different bitters recipes. Here’s one for Orange Bitters:
A drachm, as defined by the Oxford Dictionary is “a unit of weight formerly used by apothecaries, equivalent to 60 grains or one eighth of an ounce.”
60 O.P. refers to rectified spirit that is 60 parts over proof. Which explains why recipes in the Mixing Master that include Orange Bitters only call for a dash or two. A little definitely goes a long way!
For a more detailed look at Bitters, check out this page from the ultimate rabbit-hole of information, Wikipedia.