Of course, you should already know that alchemy was reckoned capable of producing a great medicine to heal all illness, or even, if you are Roger Bacon, extend your life.
Distillation was almost certainly spread around Europe by alchemists, but adopted by a variety of medical people in the 14th century, and the popularity of spirits of wine, i.e. high strength alcohol, is proof of that.

Arsenic, usually the sulphide, is found in the earliest alchemical recipes. It was also used as a medical ingredient at that time. Dioscorides, writing in his Materia Medica of the 1st century AD, used it as an astringent antiseptic, but mentions that it makes your hair fall out. The Physika et Mystika, the earliest known alchemical text, uses it to whiten copper or do various other strange things to metals.
Now, this new book of mine is a translation of a mid 15th century collection of medical recipes. And what fun it is.
The first one to draw to your attention is number 639, on page 201:
“Another for the same (nits in the head). Take quicklime and orpiment, and make powder of them; and temper the powder with vinegar, and anoint the head therewith. And this destroyeth them without falling of hair or any other harm.”

I suppose the orpiment, that is, arsenic sulphide, would kill the nits. But to claim that it doesn’t cause your hair to fall out is interesting, given what Dioscorides wrote. Taken externally I suppose it wouldn’t be absorbed as much as if taken internally. I certainly wouldn’t try the recipe myself, although I wonder how much damage preparing it did to the physician. Quicklime is calcium oxide, used for building mortar after you add water, when it becomes slaked lime.

Another recipe, number 447 on page 149 is a bit different:
For to do away with hair forever. Take four ounces of slaked lime and let it stand all night in a quart of water; and on the morrow boil it on the fire and put thereto one ounce of powder of orpiment; and thus shall thou know when it is [boiled] enough: take a feather and put it into the pot, and if you may slip the feathers from the stalk, then it is enough, otherwise not. And when thou shalt avoid (remove) hair, thou must be in a hot place, and then anoint thee with the licour, and le[it] rest awhile; and afterwards wash it in hot water, but look that the former licour not be too hot nor too cold for [fear] of stripping the skin. And use this thrice, and it will void [the hair].

Wait, now a similar composition is going to strip the hair away? This does not compute.
The internet suggests that quicklime was indeed used as a depilatory, although I have not found an authoritative source (The internet doesn’t know everything, especially if you don’t have journal access). Hair loss through arsenic poisoning is to be expected, but this is just a solution applied to the skin and probably less likely to make it occur, so I suppose the real question is, how much of a difference is there in the solution used in order to not make you go bald at the same time as killing the nits off. After all if you are bald they don’t have anywhere to live, but that would be rather an extreme reaction.

The answer is likely in the vinegar. Adding vinegar to the solution would neutralise the caustic properties of the quicklime, making calcium salts which would probably make a nice gloopy paste to spread on your head. Not that they knew about the differences between acids and alkalis then, but it is interesting that the instructions are to temper the quicklime with the vinegar. A better understanding took until the 17th or 18th century. The question which always comes to mind is, how did they work out these similar yet somewhat different recipes? All we can really is that trial and error and experience probably counted for a great deal, as well as the prevailing theories of matter, i.e. the four elements etc.

The recipes are from “A Leechbook or Collection of Medical Recipes of the Fifteenth Century”, transcribed into modern spelling and notes etc, by Warren R Dawson, published in 1934. Basically the original was written out around 1443 or so, a huge compendium in rough alphabetical order of hundreds of medieval cures. Many use herbs. Some refer back to a famous physician of the past, such as Galen or Peter of Spain.
More recipes to come!

Oh, I nearly forgot. Here’s something from the Libellus de Alchimia on arsenic:
“Arsenicum is a subtle substance of a sulphurous colour and occurs as a red stone. By nature it is like auripigmentum: the flowers are white and red. It is easily sublimed and is whitened in two ways: through decoction and sublimation.”
The red stone being arsenical sulphides.