Poison is the quiet weapon, and as such, the use of it in a crime story plot makes for some of the best crime writing there is.
If you want to write a poisoning that holds up, the trick is understanding how these cases actually get solved, because that’s where your suspense is hiding.
One important note before we start. Everything here is for the fictional page. I write about forensics so fiction can feel real; not how to commit the perfect murder. I want want you to know how poisoners get caught and how a good examiner reads a body.
You’ll find plenty of craft here and nothing that belongs in anyone’s kitchen.
The quiet weapon and why it fascinates us
Blunt force and bullets are violent expressions of harm.
Poison whispers. It can look like a heart attack, a stomach bug, a slow decline, or a sudden collapse over dinner, and that ambiguity is exactly why readers lean in. It’s also the thread that ties my whole September blog series together. The case I wrote about earlier this month, the Lizzie Borden murders, had a quiet poison subplot hiding inside it.
The day before those killings, Lizzie tried to buy prussic acid, a form of cyanide, and got turned away. She ended up using a hatchet– a much more violent way to kill a person. Her poison attempt would have been so much easier, less messy, and less strenuous.
How a poisoning is discovered
Here’s the part a lot of fiction gets backward. A body doesn’t arrive at the morgue with a glowing label that reads “arsenic.” Toxicology is a deliberate, sample-by-sample process, and it leans hard on the examiner knowing what to hunt for in the first place. The medical examiner and the toxicologist read the whole picture, what’s at the scene, the medical history of the victim, the way the illness presented, and the findings at autopsy, and those clues tell them what to test for.
From there the lab works with specific biological samples. There are a handful of bodily items that undergo testing: blood, urine, stomach contents, and tissues such as the liver, heart and kidneys. Each one tells a slightly different part of the story. Blood can show what was circulating near the time of death, the stomach can point to a recent dose, and the liver holds onto a lot of what passes through it. A good toxicologist reads them together. The same lab notes that poisons “typically require an experienced and well-trained eye to identify,” which is the whole ballgame, and I’ll come back to why.
The myth of the untraceable poison
Every writer who wants to use poison as a weapon, wants the perfect, untraceable poison. Let me be gentle but firm about this one. The classic weapons of the golden-age mysteries, arsenic, cyanide, and strychnine, are exactly the compounds a modern toxicology lab is set up to find.
All three among the substances its scientists routinely test for. Once a lab knows to look for a known poison, today’s instruments are very good at turning it up. So the flawless, undetectable toxin is mostly a fantasy… by today’s standards anyhow.
A more realistic version of “untraceable” is “undetected,” substance (aka poison) is a question of circumstance, rather than chemistry.
Room to play
A toxicology lab will only screen for a basic panel (as above) OR what it’s told to look for. The lab is superb at confirming a substance once that substance is on the list. However, that list has to come from a human being telling the lab what to test for.
Think about that in the hands of you, the storyteller.
A poison might slip through because nobody orders the test for it.
Or nobody orders the test because the death looks natural.
If a body is buried before anyone thinks to test for a substance, that substance could break down in the body over time. Or, if the body is cremated, there is nothing by ashes to test.
Your killer’s real advantage isn’t some exotic toxin. It’s a convincing, ordinary explanation. And your detective’s real victory is the moment of suspicion that leads her to say, “Test for this, specifically.”
Getting the details right on the page
A few habits will keep your poisoning plot credible:
- Match the poison to the symptoms and the clock. Different substances act at different speeds and leave different marks. Readers who know the genre will catch it if a fast poison produces a slow, wasting death. Decide the effect the story needs, then let the substance behave the way it really would.
- Give your investigator a reason to suspect. Since detection follows suspicion, plant the clue that trips the right test. An out-of-place symptom, an offhand remark from a witness, a poisoning in the victim’s past.
- Respect the samples. If your examiner is thorough, let them draw blood, hold tissue, and keep the stomach contents for testing at a later date. If your plot needs the poison missed, give a concrete reason those samples were never taken or never tested, and let that gap cost somebody something later.
Ratchet Up The Suspense
Get the science straight and the suspense mostly takes care of itself. A poisoning story is a duel of knowledge. Your killer is betting nobody will think to look, and your investigator is fighting to find the one thing she needs to prove the poison, before the trail goes cold.
If you want to go deeper into the forensic world… evidence, cause and manner of death, and how a medical examiner really works a case, take a read of my Coroner’s Daughter series, where I make it my mission to use factual forensics in fictional storytelling.
I break the real science down for writers in Forensic Speak.
And… I dig into how to use poisoning, or any other type of murder, to created riveting plots in my upcoming master course, Writing the Killer Crime Story, launching soon. Watch the newsletter for the release date. Join the waitlist, and I hope you’ll see me soon!
Stay curious. Jen
