No Stupid Questions

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.

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.

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.

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).

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.

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.

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

Thallium was pretty famous for this until its mechanism of toxicity and antidotes were better understood. Slow acting, tasteless, odorless, colorless, symptoms weird and mimic other things. Used to be used for rat poison but the risk of accidental exposure was too high. Requires late 1800s technology.

Source of radiation given to someone to hold in their wallet/pocket

Am not sure if helpful, or if this would be somewhere you could visit for reference.

The Alnwick Victorian poison garden is fascinating and perhaps even has ideas for you.

https://www.alnwickgarden.com/the-garden/poison-garden/

https://en.m.wikipedia.org/wiki/Alnwick_Garden

Hope this helps. Am hoping to get to visit one day.

Go ask chatgpt or something similar for old medical herbs used back in the day that might also posion you. I inspire to write a book one day, and ive found ai to be really informative. The best part is, even if they're wrong its going in a fiction story(i hope) where it wouldn't matter.

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.