Bittermint was a poison that could be mixed with alcoholic beverages. It was brewed like tea in boiling water from a secret mixture of mosses, fungi, and samples of bark from certain trees or shrubs.
This colored, watery liquid had a faint, bitter taste of mint, for which it was named. The strength of the effect depended on the concentration consumed and now long it had been mixed with alcohol. Time and dilution both reduced the effects.
Unless the imbiber resisted the poison, they quickly felt like they took a punch in the stomach and eventually started to lose coordination in their limbs.
The managers of the Moon Mountain Brewery, Yarimmur Borulglar, Kelter Minstrelwish, and Nurvala "Mother" Minstrelwish, secretly brewed this concoction for the Zhentarim and, when ordered to do so, added it to barrels of their ale, rum, or hard cider that were bound for a particular victim.