what is saltpeter used for
Magically speaking, Saltpeter has a long, long, long history in hoodoo as an agent of magical protection in spritual baths and floor washes and as a shoe-sprinkle. Also, because it is an ingredient in Gunpowder, it is used in formulae which are hoped to produce quick (explosive) action, such as Block-Buster.
Cecil replies: The official word is that potassium nitrate (KNO3), more commonly employed as an ingredient in gunpowder, has no therapeutic value as an anaphrodisiac, contrary to legend. Cecil of course believes this. Still, when you look at what the stuff does do, you can see where the idea got started. Saltpeter can cause relaxation of involuntary muscle fiber (for which reason it's used to treat asthma) and it's occasionally prescribed to lower body temperature in cases of fever.