No Stupid Questions

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.

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.

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.

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

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).

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

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

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.

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.

• 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.

Whole maybe not “poisons” by definition I have a couple scary stories of people working in damp, moldy office and basement environments and after a couple years getting rare autoimmune and neurological disorders that killed them. One being my uncle, my family tried to get his workplace to test where he worked because the doctors said that’s most likely where he contracted it, but they refused. We weren’t looking for money, just trying to save the next guy. I was fairly young when this happened so I don’t remember all the details.

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.

I'm a writer

"A writer" 😉