The witches’ ingredients for the cauldron in Macbeth are mostly coded names for medicinal plants.
Macbeth’s Act 4 opening scene is probably one of the most well known scenes in Shakespeare. The Three Witches (aka the Weird Sisters) are in a dark cave with a boiling cauldron at the center. The witches gather and begin an incantation of dark magic, adding ingredients as they go. “Double, double, toil and trouble; Fire burn, and cauldron bubble.” Each witch then lists a series of ingredients that, at face-value, sound disgusting and macabre. But the ingredients are not what they seem.
Witches, like early alchemists, kept their knowledge a closely guarded secret. After putting all of that effort into R&D you didn’t want the competition getting a hold of your findings. But witches had other reasons for secrecy. Unlike the fictional witches in Macbeth, most “witches” were in reality female medical workers using herbs & botanicals to create medicines. In Medieval Europe medicine was a male-dominated profession and women caught practicing medicine were sometimes branded as witches. As such they had to keep their activity, and their recipes, secret. By using alternate names for their ingredients, the contents of one’s “spell book” were kept secret. Also, using alternate names worked as a safe-guard to keep their proprietary medical knowledge out of the hands of the general public who might mistakenly try to prepare these treatments themselves.
Some believe that almost all of the 23 ingredients in the witches’ brew are actually coded plant names, even the problematically named “liver of blaspheming Jew”. How much of this Shakespeare knew is unclear. All the ingredients together would make an unrealistic ridiculous concoction, so they were probably chosen for theatrical shock value. He may have found a list of real “witches” ingredients and used them without knowing they were coded names. Still, if you want improved blood circulation some eye of newt (aka mustard seed) might help.