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.

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 properties you want/need. Now excuse me while I continue to work on my immunity to iocane powder.

Alcohol. Sometimes it takes 30 or 40 years to be effective. Not very good for murder, but wildly popular for suicide.

I’m neither a writer nor a scientist, but there’s a copy of this on my bookshelf and I wish I could lend it to you: Deadly Doses: The Writer’s Guide to Poisons

I'm a writer

"A writer" 😉

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

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.

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).

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.

how slow acting? As in months, or as in time to get the fuck outta dodge?

Ricin will kill you in a couple of days, was first extracted in 1888, and the plant grows in the mediterranean.

Depending on your setting and desired outcome for the poisoner, uraninite (aka pitchblende) might be an option. It has historical uses in glass making and pottery glazing, which could provide justification for why someone would have it.

It contains Uranium, which is radioactive, but I don't believe will bioaccumulate, but can build up on surfaces, tools, and clothing providing a source of long-term radiation exposure. In addition, it contains lead, which does bioaccumulate, providing a source of gradual long term poisoning as well as radium which also bioaccumulates and is radioactive, providing an additional source of longterm radiation exposure.

I've heard something about writers writing about guns -- if you describe a specific gun they'll come at you for being wrong, but if you say something like "a modified Kalashnikov" you're conveying the image you want, and the nerds will do the work for you in figuring out how it could have been modified.

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.

I'm thinking metal poisoning over time. Lead or copper, for example.

Apple seeds contain cyanide. You'd have to crush and eat anywhere from 150 to a few thousand seeds for it to be fatal though. I'm sure that hasn't stopped authors from using it before.

Mistletoe berries i think. Otherwise mercury depending on dosage.

For that i think having a read about acqua tofana would be a pretty good source of inspiration, since it was designed to have effects like an illness. it originates in italy so most of it's ingredients are by default from a mediterran climate.

Or if you want something natural without too mutch human processing, belladonna is a surefire, but not slow acting.

• Heavy metals: lead, mercury
• Arsenic in small doses over a long period

I don’t know of any plants, but I do know that the leaves of nightshades (potato, tomato, eggplant, capsicum, tobacco) are ~~poisonous~~ toxic in large doses.

Arsenic is a classic murder poison. It's been known since anciemt times, though possibly unsuited to your onset requirement. Acute poisoning by ingestion is generally within a few hours, but if your character sustains lower doses over time, you could probably draw out the timeline to whatever you wanted. It would be obvious that the character is unwell during this time, but the symptoms aren't super specific and could be confused with e.g. food poisoning.

Or just invent a mushroom like others said. The toxins are diverse enough that I doubt anyone would be too upset if you tuned it exactly to your timeline and desired symptoms.

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

How slow are you talking? Days? Weeks? months?

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.

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.

Mushroom poisons usually are slow actors. They take a few days while they kill your liver, and then you'll follow suit.