You could go for Paracetamol/Acetaminophen. The lethal dose is quite low and in theory low enough someone could poison someone else with it. And once symptoms set in people’s livers are often beyond rescue and they die a very gruesome death unless we find a transplant organ in time.(And even then survival is not guaranteed)

And it’s relatively save to use in writing as it is coated/mixed with enough bittering agent these days that it actually wouldn’t work that well to secretly posing someone.

If you need something with a shorter timeframe Methanol is an option.

And of course there’s always Dihydrogen monoxide-everyone who has even had one drop of it will die eventually but the time range depends on the dose. With very high doses people die in minutes,with medium doses (this is actually sometimes used by inmates to kill themselves) they die within a day, with lower dose after decades, but some die mere days after they ingested the mere last drop of it. Nasty stuff and very available.

Mushrooms are a good option, and you can just make up a species if you want specific time frames/symptoms. Mushrooms can cause a lot of weird symptoms.

There’s also a brain eating amoeba or other sickness from still water (people back in the day were very aware of tainted water).

If you have access to polar animals, a unique poisoning would be vitamin A toxicity from their livers. It’s a horrific way to die, though (skin sloughing off).

For what it’s worth, I was able to get some AI suggestions. You’ll ofc have to research these more yourself but it could be a starting point for inspiration. Online models won’t answer this but I was able to persuade my local model to give me a list. The following is from the llm:

For a poison that can be extracted without modern technology and fits your climate criteria, I'd suggest the 
following options:

1. **Aconitum** ( Monk's Hood or Wolf's Bane): A toxic plant native to Europe, Western Asia, and North Africa, 
which thrives in temperate to Mediterranean climates. The plant contains aconitine, a powerful neurotoxin that can 
cause numbness, nausea, and eventually death if ingested.
2. **Henbane (Hyoscyamus niger)**: A perennial herb native to Europe, Asia, and North Africa, commonly found in 
temperate and Mediterranean regions. It contains scopolamine and hyoscyamine, which can cause hallucinations, 
delirium, and paralysis if ingested.
3. **Yew (Taxus baccata)**: A coniferous tree native to Europe, Western Asia, and North Africa, often found in 
temperate and Mediterranean climates. The seeds of the yew plant contain taxine, a toxic compound that can cause 
cardiac problems and death if ingested.

For a more subtle, slower-acting poison, you might consider:

1. **Oleander (Nerium oleander)**: A shrub native to the Mediterranean region, which also thrives in temperate 
climates. The plant contains oleandrin, a cardiac glycoside that can cause arrhythmias and death if ingested.
2. **Datura** (Jimsonweed or Devil's Trumpet): A flowering plant native to the Americas, but introduced to other 
regions, including Europe and Asia. It grows in temperate climates and contains scopolamine and hyoscyamine, which 
can cause hallucinations, delirium, and paralysis if ingested.

Keep in mind that these plants are all highly toxic and should be handled with caution (if at all). If you're 
writing a scene where someone gets poisoned, it's essential to convey the severity of the situation without 
sensationalizing or glamorizing the poison.

Feel free to mix and match or modify these options to fit your story. Good luck with your writing!

Lead, mercury, and other (mostly heavy) metals. Look up mad hatters. Also phosphorus and match girls

Dimethylmercury.

Two drops will kill you in a few months and nothing can be done. It penetrates clothing, regular latex gloves, and skin very easily and unnoticed.

Pretty terrifying stuff. Not sure you’d be able to figure out availability or production in your plot, but as far as poisons go you’d be set for time between dose and death.

Ricin was used in Breaking Bad. Fairly easy to produce and administer.

Yeah, in a historic setting, use something readers will recognize, as well. Arsenic, Mercury, that kind of thing. They’ve been used as a poison, and have accidentally poisoned, for so long that they’re tropes of their own. Both of those in specific were available in the region you’re using.

Plus, they’re going to be really easy to describe the actions of, and don’t require medical knowledge to understand the effects of. Well, the stuff that’s going to be useful to show on page anyway, the stuff that happens inside organs might take a little.

As a writter you should get enough details wrong that someone trying to follow your recipie fails. Ideally they are also caught.

Alcohol is a very effective slow acting poison.

There was a theory a while back that Napoleon died from arsenic poisoning due to the damp Mediterranean climate causing the decay of a green wallpaper using an arsenic based dye. It didn’t end up being entirely true but he may have been slowly poised by environmental arsenic throughout his life. Here is an article about it.

Arsenic is a naturally occurring element in the Earth’s crust as metallic crystals or combined with other elements. Being a heavy metal, arsenic bioaccumulates. That is, it builds up in the body over time, so slow poisoning is possible.

Heavy metals in artists’ oil paints.

You can always dig into true crime, see what methods are used to kill husbands.

Check out ChubbyEmu on YouTube. He’s a pharmacist who presents strange and interesting medical stories (with dramatic re-enactments) involving unusual chemical exposures that damage and sometimes kill people. Examples: soda, coconut water from a bad coconut, a fermented soup, ivermectin ordered online, lab chemicals stolen by a student and given to a disliked roommate, and so on. Maybe something there will inspire you. ChubbyEmu does a good job of breaking down complex medical into an easily digestible format.

What do you mean by slow? Time till symptoms? time till death? Hours? Days? Weeks?

Raw castor beans contain ricin its a cool looking plant that gets big. My neighbor grew on once accidentally.

symptoms commonly begin within two to four hours, but may be delayed by up to 36 hours.

Unless treated, death can be expected to occur within 3–5 days; however, in most cases a full recovery can be made.

Actually a lot of beans are toxic when raw, but not deadly. Raw lima beans are special though; they contain something that the human body breaks down into cyanide. No clue how long that takes or how many it would take to cause harm.

Heavy metals (mercury, arsenic, cadmium, lead) are known for building up over time with many exposures. Think mad hatter syndrome, etc. but exposures can also be acute if high enough. IIRC acute arsenic poisoning makes you vomit and diarrhea until you die of dehydration after days or weeks.

Good luck with your writing! I too have had to search for some very silly things for things I’ve written! As for slow acting poisons, only ones I could think of that would be really available modernly would be like, antifreeze and such, but I don’t think that’s so slow acting. And usually it’s kids and dogs that get into that.

Time. It’s very slow but will get them in the end.

In almost every case in fictional writing it’s better to make up a poison then use a real one. That way you don’t have someone picking it apart later. Also you can give it whatever propties you want/need. Now excuse me while I continue to work on my immunity to iocane powder.