It occured to me, when I had a story idea set in 15th century London, that I needed to know about medieval poisons. Unfortunately finding good information was not very easy, the internet being full of rubbish as usual, and there not apparently being many books on the topic.

At first glance, turning to “The Goodman of Paris”, a late 14th century book on how to run a household, is not the best way to start. Fortunately it has a section on poisoning rats and suchlike. Page 139 of the Dover paperback has instructions for a powder to kill wolves or foxes. It contains the root of black Hellebore, dried, powdered, and mixed into it a 5th part of ground glass and a fourth part of lily leaf.
You then take honey and fresh fat in equal quantities, mix them with the powder, and make it into a paste and balls the size of a hens egg. These are covered in more fresh fat and laid where the wolves will come and eat them. Or you can scatter the powder onto dead animals that the foxes etc are going to come and eat.
What effect might these ingredients have?

Using wikipedia as a reasonable first source, we find that
Hellebore causes tinnitus, vertigo, stupor, thirst, swelling of the tongue and throat, vomiting, slowing of the heart rate and finally collapse and death from cardiac arrest. Sounds bad.
Aconite is another plant that is toxic, causing gastrointestinal problems, vomiting and diarhoea, with other symptoms such as sweating, dizziness, difficulty breathing etc. Eventual cause of death is heart arrythms and paralysis of the heart or respiratory centres.
Lily leaf – well that all depends on what kind of lily they mean. The Lly of the valley is a common plant in Europe and has toxins all the way through it, which affect the heart the same as the two previous examples.
Ground glass was thought of as a poison all the way into the 20th century, but it isn’t really. It might irritate your GI tract, but you, sorry, the animal would have to ingest entire shards of it to cause any possible trouble.
There’s no real concept of the dose of the poisons involved, but I expect it would be fairly effective, just not every time and it might depend on how well you mixed it all and how fresh everything was. Perhaps the alkaloids are fat soluble, which would surely help with their absorption and transfer to the animal.

For rats, you catch them by using cats, or people employed to catch them, or make traps, or finally making cakes out of paste, toasted cheese and aconite and leave them near their holes. You can also cut up sponge small, for them to swallow and then when they drink the sponge is supposed to swell inside them and kill them. Given the dangers of aconite I expect this would be fairly effective, or else make them ill enough that they are easy prey for cats or terriers.
Another recipe says take an ounce of aconite, two ounces of fine arsenic, a quarter of pigs fat, a pound of fine whaten meal and four eggs, and make this into a bread which you cook in the oven! Okay, the arsenic will be the sulphide and probably not contaminate the bread oven much, but that is one recipe I’d rather not try. It’s also rather an expensive one, using enough food to feed someone for a day, although it also shows that arsenic was available to buy in big towns, as you would expect given that physicians used it in medicines and alchemists used it in their processes.

Examining my copy of “A leechbook or collection of medical recipes of the fifteenth century” by Dawson, I find no obvious poison recipes, as you would expect – a physician was interested in curing illness, not causing it. There are recipes for powders that cure poison or venom, and they follow the usual herbal path. Whilst the author or compiler was aware that arsenic compounds could make the hair fall out, there is a recipe which specifically says it avoids that, using arsenic, lime and other stuff, so it seems they were aware of the dangers of it. But their approach to risk is very clearly not modern, rather more like it was a hundred years ago, something that would require its own blog post to discuss.

When we turn to Volume III of Thorndike’s massive history of magic and experimental science, we find an entire chapter on poisons. The important thing to note is that they were thought of as working due to “occult virtue”, i.e. they worked by similarity, by being like something, rather than by chemical or other means. Unfortunately Thorndike doesn’t mention much about the specific poisons used.

An SCA source which has been useful in the past has a page on the topic:
Petri de Abano, who in the 1300’s listed mercury, gypsum, copper, iron, rust, magnetite, lapis lazuli, arsenic sublimate, litharge, lead, realgar, cateputria, cucumber, usnea, coriander, hellebore, mezereon, fool’s parsley, bryony, nux vomica, colocynth, laurel berries, cicuta, serpentary, and cantharides as poisons in his work, _De Remedis Venenorum_.  Similarly, Magister Santes de
Ardoynis mentions arsenic, aconite, hellebore, laurel, opium, bryony, mandrake, cantharides, leopard’s gall, cat’s brains, and menstrual blood among the poisons in his _Book of Venoms_, written in 1424.[a]

Now at least there’s some lists there, useful for anyone writing a medieval story, although it would surely help if the poisoner could read or knew a pharmacist. Some of these substances just aren’t dangerous, but as you would expect, there was a lot of hearsay involved; the lists were probably compiled from current stories and old manuscripts. Some of them were known as poisonous in ancient Rome and Greece, and that knowledge was passed down through the writings of Dioscorides and others.

More information on alkaloids and the history of their use can be found here: