It's made from water, alcohol, gentian root, and vegetable flavoring extracts, this type of bitters is often called simply angostura, and is used mostly for food and beverages. It is produced by the House of Angostura in the country of Trinidad and Tobago, and despite having an alcohol content of 44.7% this concentrated bitter is still not classed as an alcoholic beverage in the U.K. in line with laws on other bitters. Angostura is easily recognisable due to the bottle having an oversized label.
The recipe was first developed to be used as a tonic by a Surgeon General in the army of Simon Bolivar in Venezuela called Dr. Johann Gottlieb Benjamin Siegert. Johann was based in Ciudad Bolívar which was then known as Angostura, and used locally available ingredients to manufacture and start selling the bitters in 1824. The exact formula is a closely guarded trade secret, with only five people in the world knowing the whole recipe.
Angostura bitters are so concentrated that they are normall not drunk purely, but instead often used as a flavoring for drinks and food, and even then, only a few drops or dashes are enough. Angostura bitters are a key ingredient in many cocktails, originally used to mask the flavour of quinine, which is extremely bitter, in tonic water along with gin, this mix stuck in the form of a Pink Gin, and is also used in many other cocktails such as Long vodka, a cocktail consisting of vodka, angostura, and lemonade.
It is believed that Angostura bitters have restorative properties, reportedly being a remedy for hiccups and for an upset stomach. It is also often believed to have poisonous qualities because it is assumed that they contain Angostura bark, which in itself is not poisonous, but when it was sold as a medicine it was often sold by unscrupulous sellers who padded out the sacks of bark with poisonous barks to make more profit. The bitters does in fact not even contain the bark.