Does anyone know of any compound/substance one could have access to in a stone prison cell (From food? The rocks? The metal bars?) that could be processed through rudimentary means into a quick acting poison/venom?


  • That it can quickly (within the hour) start showing effects (whether ingested or applied into a wound).
  • Can't be made through means unavailable to an ordinary person inside a prison cell.
  • It can be something that doesn't cause death but instead leaves the person heavily impaired (Paralysis, loss of consciousness, stroke-like symptoms...).
  • Time is not a problem, production can take up to a month, and it only needs to be a single dose.
  • It has to be a small dose.
  • Can be extracted from food, but can't come from "specific herbs", the character is imprisoned and as such can't go out to look for specific plants/minerals.

In the story I'm writing, the protagonist finds themselves locked up and enslaved in a medieval prison/castle. Though there is magic in the setting it is not available to him.

My current idea was that he gathers apple/apricot seeds and crush them to make a cyanide-based poison, but by the looks of it there's no way to make it concentrated enough for it to be a relatively small dose.

  • $\begingroup$ Do you expect the guards to care about a sick prisoner, in view to escape, or to poison a guard? If the first, have you considered eating straw from your mattress? Or dirt? After one pound or two, you'll become very sick. $\endgroup$ – kikirex Mar 20 at 0:17
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    $\begingroup$ @kikirex the intent is to poison someone very important that happens to go through the prison area every now and then, hence why it has to be a small dose. And it has to be relatively quick because if they start showing symptoms once they get outside the prison then it's likely they'll get a healer to look into their situation. $\endgroup$ – Mario Garcia Mar 20 at 0:22
  • $\begingroup$ Required study material: Good Knight MacGyver: Part 1, Part 2. MacGyver goes back in time, kinda, and while he does not make a poison, he MacGyvers an antidote. $\endgroup$ – pipe Mar 20 at 21:14


Ricin is found naturally in castor beans, a common plant (in various parts of the world), and extremely fatal in tiny doses. In fact, the CDC has a bioterrosim page dedicated to it

Castor beans may be used in different types of foods and/or natural remedies. Depending on the timeline and location of your story, as well as other details (such as whether or not the prisoner is forced to work in the kitchen, where s/he may have access to raw castor beans) makes this a viable option.

Although onset is a few hours, ricin is extremely fatal, there is no known cure, and has been used by real-world assassins.


Improperly canned foods can cause botulism - it would be rather easy for a prisoner with access to raw foods to prepare / hide the foods until they rot and the bacteria develops. Getting it on/in a guard without them noticing will be more difficult. Although possibly fatal, it's rare, but very debilitating.



Being in a prison is quite possibly unsavory. Quite a few fungi are poisonous and could be growing in moldering hay on the floor of the cell

There are also lichens that are poisonous which could be growing on the walls.

  • $\begingroup$ Unfortunately these take six hours or more before they show their poisonous effects. $\endgroup$ – Justin Thyme Mar 20 at 2:13
  • $\begingroup$ Being locked in a cell with no materials or equipment makes it hard. Might have to settle for a fictional mushroom. $\endgroup$ – Thorne Mar 20 at 4:51

St. Anthony's Fire

A fungus also known as ergot, found on rye grains and in rye bread containing a natural LSD substance.

The convulsive symptoms from ergot-tainted rye may have been the source of accusations of bewitchment that spurred the Salem witch trials.

Thought to have inspired the Triptych of the Temptation of St. Anthony, this is the detail of Saint Anthony being carried into the sky by demons:

enter image description here

Hieronymus Bosch - Wikipedia 2019, CCASA Licence.

characterized by muscle spasms, fever and hallucinations and the victims may appear dazed, be unable to speak, become manic, or have other forms of paralysis or tremors, and suffer from hallucinations and other distorted perceptions.... violent burning, peripheral pulses and shooting pain of the poorly vascularized distal organs, such as the fingers and toes...

If the poisoning continues long term then gangrene of the limbs and death will result.

  • 1
    $\begingroup$ This is probably your best bet. en.wikipedia.org/wiki/Claviceps_purpurea One just needs to grow rye in the prison cell. The fungi needs warm, moist conditions to grow. In high concentrations, I believe the effect is relatively immediate. It is also possible to extract an LSD-like substance from it. That would certainly get someone's attention. The spores can be carried by moths. $\endgroup$ – Justin Thyme Mar 20 at 14:10
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    $\begingroup$ 'Sclerotia germinate in spring after a period of low temperature. A temperature of 0-5 °C for at least 25 days is required. Water before the cold period is also necessary.[9] Favorable temperatures for stroma production are in the range of 10-25 °C.[10] Favorable temperatures for mycelial growth are in the range of 20-30 °C with an optimum at 25 °C' from same source. $\endgroup$ – Justin Thyme Mar 20 at 14:14
  • $\begingroup$ How long does it take to show effects after eating an ergot-infected meal? I can't seem to find sources on the speed it has. $\endgroup$ – Mario Garcia Mar 20 at 22:36
  • $\begingroup$ @MarioGarcia For LSD a few minutes, for ergot I tried to find a figure or case study, but couldnt. I'd suggest, if ground-up and given in a drink there shouldn't be a barrier to it's effect being within a couple of minutes - if the subject hadn't just had milk or something that coats the stomach lining. Could be worth making a question for Medical Sciences. $\endgroup$ – Confounded by beige fish. Mar 20 at 22:44

Leave your food for a month alone. It will either rot or develop a fungus. You dry the fungus and mill it. If it will rot you just save the "juice" and use as a poison. In both cases you need to create concentrate for it to start acting quickly.

If your prisoner get feed apples he can create cyanide form apple seeds. Downside to this is that 80kg person need to consume 250 mg of cyanide to show symptoms as you described. That equal roughly to 6000 apple seeds. Apple can have max 20 seeds in one. So In the best scenario he would need 300 apples to create enough cyanide to poison certain person (with death as a possible outcome if he get the weight wrong).

There is not much to "get" from prison in your given time bracket. Botulism need at least 12 hours to develop. Regular food poisonings (salmonella, rare meat etc) need 24. anything that is widely available is not poisonous because if it's occurrence in the world is common people have natural immune to them. As you build immunology to certain poisons by taking small doses of it.


Spider bite.

I knew the Australian funnel-web spiders were bad, but I did not know that their venom acted so fast!

Envenomation symptoms observed following bites by these spiders are very similar. The bite is initially very painful, due to the size of the fangs penetrating the skin.

Early symptoms of systemic envenomation include goose bumps, sweating, tingling around the mouth and tongue, twitching (initially facial and intercostal), salivation, watery eyes, elevated heart rate, and elevated blood pressure. As systemic envenomation progresses, symptoms include nausea, vomiting, shortness of breath (caused by airway obstruction), agitation, confusion, writhing, grimacing, muscle spasms, pulmonary oedema (of neurogenic or hypertensive origin), metabolic acidosis, and extreme hypertension. The final stages of severe envenomation include dilation of the pupils (often fixed), uncontrolled generalised muscle twitching, unconsciousness, elevated intracranial pressure, and death. Death generally is a result of progressive hypotension or possibly elevated intracranial pressure consequent on cerebral oedema

The onset of severe envenomation can be rapid. In one prospective study, the median time to onset of envenomation was 28 minutes, with only two cases having onset after 2 hours (both had pressure immobilisation bandages applied).

Funnel web males apparently wander around certain times of the year, looking for mates. That is how they run afoul of humans. In your world there is a similar spider (or some other small invertebrate - scorpion might be good) and these things turn up in the prisoner's cell from time to time. Your prisoner knows what they are and after killing a couple, decides to keep one as a pet, feeding it lice from his own body. When the target shows up he surreptitiously flips the creature onto him. The spider seeks refuge in the target's clothes and bites him a minute or two later.

This seems to me more plausible than some ingested poison. If I am an occasional visitor I am not going to have a prisoner make me an omelette or give me a tattoo. I might come close enough to get a spider flicked on me.

If these things live in the prison the guards will probably know what is going on when the target starts gets sick. But an aristocratic target might not deign to tell lowly guards he is feeling poorly, and instead consult with his own entourage or wait to get help from a healer appropriate for his class and station. That would make good narrative, as one of the guards suspects the target has been bitten but then is ignored / dismissed.


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