Menstruum is an old word meaning solvent, and in herbal circles it refers to the substance (usually liquid) you use to extract certain properties from herbs, thus making a medicine.
The two menstruums most people are familiar with are water, and spirit alcohol (tinctures and extracts you buy in a health food shop are made with alcohol). But you can also make medicines with vinegar, honey, or oil/fat. Here’s an overview, and I’ll do separate posts in more detail on each menstruum.
Water: used for making teas, infusions, or decoctions for internal use, and compresses or foments for external use. They are used as is or as the base for further medicine making. The terms are used interchangeably and contradictorily by herbalists, so it pays to check what any individual actually means. Water is good for extracting minerals, vitamins, and other water soluble parts like tannins or mucilage. Usually hot water is used, but sometimes not.
Alcohol: used for making tinctures and liqueurs, or liniments for external use. It extracts properties that water can’t, and tends to produce a stronger medicine than water.
Vinegar: used for herbal vinegars. It is excellent at extracting minerals, so can be used to make a mineral supplement. Can be used to make tinctures for people that avoid alcohol but is not as potent as alcohol.
Oil: herbal oils are for external use, either as is or for making ointments or salves. Fat is the traditional menstruum but many people now use a vegetable oil.
Honey: a divine menstruum, virtually any tasty herb can be put up in honey for pleasure, food or medicine.