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The saying goes "in dose venenum", meaning that it's the dose to make the poison.

This comes from the consideration that any substance, after a certain dose, becomes lethal. However, past the minimum lethal dose, the substance will stay lethal.

I am toying with the idea of a two thresholds poison: this substance is lethal only above a dose X and below a dose Y, with Y>X>0. In all other dosages it is not lethal. In symbols, 0 < X < lethal dose < Y

Is there any biological, chemical or physical mechanism which can explain the above behavior? Or should I just explain with handwavium?

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    $\begingroup$ I don't think that such a chemical exists. Usually if you reach the dosage in which it becomes poisonous, increasing the dosage usually just makes it worse. My best approach at this would be an analogy to an inuit diet transition, in which your body undergoes changes to adapt to their diet rich in fat and relatively poor in sugar and begins to burn down fat more easily,but until they adapted, that extra fat consumed might be a problem for them. I see something similar here, with a high enough concentration inducing relatively fast metabolic changes which somehow nullify its poisonous effects. $\endgroup$ – ProjectApex Jul 2 at 10:50
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    $\begingroup$ I feel like my answer to Can a substance be more lethal in smaller doses? would answer this perfectly. I don't think this should be closed as a duplicate of that question, but I'm wary of reposting the exact same answer. $\endgroup$ – F1Krazy Jul 2 at 11:18
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    $\begingroup$ Simply make too much poison cause the imbiber to throw up, expelling the rest of the poison. $\endgroup$ – Dragongeek Jul 2 at 11:26
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    $\begingroup$ Do you consider zero amount of the substance to be in the "below threshold" interval? In that case, my answer would be oxygen. $\endgroup$ – Radovan Garabík Jul 2 at 11:47
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    $\begingroup$ @RadovanGarabík, no, it has to be a non zero dose. But actually 0 oxygen also kills... $\endgroup$ – L.Dutch - Reinstate Monica Jul 2 at 11:48

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As noted, I feel like my answer to a previous question will also apply here.

What you're looking for is an emetic: a substance that induces vomiting. Your X threshold remains the point at which a dose becomes lethal, but the Y threshold becomes the point at which the emetic properties kick in and you throw all the poison back up before it can enter your bloodstream, thus saving your life.

While there's no real-world substance that's fatally poisonous below the dose at which it induces vomiting, you can simply invent your own, and just tweak the X and Y thresholds until they're just right for your story.

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    $\begingroup$ +1, this same poison concept appears in Asimov's novel The Naked Sun. $\endgroup$ – Carmeister Jul 2 at 19:53
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    $\begingroup$ I had a similar thought. At below a certain level, it slowly kills you by building up in your system and not getting removed by the kidneys, etc. in a timely manner. But at a certain level it induces vomiting if you ingest too much at once so it doesn't have enough time to be absorbed much. And it doesn't induce vomiting through the slow method, since it's not in your stomach. However, if you continue to ingest it, or ingest a large enough portion at once, there is still enough absorbed that you're still dead. $\endgroup$ – computercarguy Jul 2 at 22:10
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    $\begingroup$ @Carmeister nice reference. Had to go back on the book to check that, i had forgotten that detail. $\endgroup$ – bracco23 Jul 3 at 9:14
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    $\begingroup$ "In real-life, many pills are actually coated in a small amount of emetic for this very purpose" out of curiosity, could you provide a reference for that ? It would probably help the OP too. $\endgroup$ – Vincent Fourmond Jul 3 at 9:48
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    $\begingroup$ @VincentFourmond Sleeping pills are in this category. It is very hard to OD on sleeping pills because they make you throw up. Their bitter taste is a property of the emetic substance. There's a reason you do not find sleeping pills in gel capsules. $\endgroup$ – Nelson Jul 4 at 5:31
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One possibility would be a mixture of a poison that binds to a cell receptor and a lower concentration of a non-poison that binds more strongly to the same receptor (i.e. an antagonist with higher affinity).

With a small dose, the non-poison only binds to a fraction of the receptors; the poison binds to the remaining ones and the target dies.

With a large dose, the non-poison binds to all the receptors and blocks the poison, so the poison has no effect.

This is more or less how heroin and the antidote naloxone work; the naloxone binds strongly to the same receptors as heroin, but has no effect itself, so it reverses the action of heroin. (At least that's what I've read; I have no personal experience.)

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  • $\begingroup$ "Asking for a friend" :P $\endgroup$ – Akita Jul 3 at 16:21
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    $\begingroup$ More seriously, this is my favorite answer as there is no spectacular effect when above the 2nd threshold $\endgroup$ – Akita Jul 3 at 16:22
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    $\begingroup$ See also: methanol and ethanol $\endgroup$ – user253751 Jul 3 at 17:08
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I can imagine a solution, but it's gross.

The poison can have two effects. One mild effect if that it will cause very mild nausea. This effect kick in quite fast, in the first minutes. The second effect is the lethal effect (paralysis of the lungs, for example, or any other), but the substance provocking this second effect is hard to digest, and it doesn't enter the blood flow before a few hours.

Below the first threshold, the body can fight the first effect and experience something between no effect at all to near death depending on the dose, the victim health, etc. Above the first threshold, the victim feels nauseous but not enough to actually give its breakfast back to earth, and die a few hours later. Above the second threshold, the victim handwave their stomach content and can expect only trace amount of poison to enter their system.

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The poison doesn't actually kill you, it just makes you thrash around uncontrollably, jump off cliffs, swim out into shark-infested waters, start fights with large predators, and take assorted other actions that shorten your lifespan.

At a high enough dose, though, you pass out, sleep it off, and wake up with nothing worse than a hangover.

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Nanites! The poison was designed by a xenophobic warlord long ago, to kill off strangers and newcomers to the defended lands. All original citizens of these lands were inoculated with large populations of the nanites long ago. When they have children, those children have large colonies of the nanites within them even from the moment of their birth. All of the fruits, vegetables and livestock in the defended lands also contain living nanites.

When a newcomer to the lands eats something (or when exported nanite-infested crops are eaten by people not from the lands), the newly ingested nanites realize that they are alone in this consumer. which is the trigger for their primary function. That function is to kill the consumer who is obviously not a native of the defended lands.

With this kind of setup, eating one defended lands apple would be fatal to an outsider, but pulverizing a dozen apples into sauce and drinking it all down at once might provide them with a large enough initial nanite population size that they can be mistaken for being a defended lands native.

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Actually alcohol tainted with methanol has exactly this effect. Ethanol is more preferred on the metabolic pathway than methanol. As long as you're sufficiently drunk, your body will be occupied with ethanol and will not try to metabolise methanol, which means methanol will exit body through urine.

However if you only drink smaller amount of tainted alcohol, the methanol in it will be metabolised into formic acid which is fatal even in small concentrations.

Sadly, this is not just a theory. In 2012, tainted alcohol has entered Czech market and to this day, has claimed 51 lives. People who drunk it with no moderation at all ended up better than those who drunk with moderation.

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  • $\begingroup$ Brilliant answer so sad that it's true. $\endgroup$ – Tantalus' touch. Jul 8 at 15:20
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pH.

It itself is highly acidic. In small doses, it's poisonous payload is activated (say the pH has to be slightly above the normal pH of some part of the digestive tract). In larger doses, it inactivates itself and passively moves through the system. Play with the numbers to get the values where you want them and in accordance with a normal digestive system. I'm sure a buffering compound could be worked in which acts as the trigger (or possibly even to prevent the poison from activating).

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Addictive Alkaloids:

Your population is all hopelessly addicted to powerful stimulants. If they don't get enough, they are listless and apathetic. A tiny dose is enjoyable. A moderate amount of poison is lethal because it alters brain chemistry but doesn't stimulate enough to have the full drug effect - their bodies will shut down and die. If they get lots, the poison shifts their metabolism and they feel invincible, smart, and happy. The drug effects outlast the presence of the poison, so they don't die as the poison leaves their body/breaks down.

A similar approach could work for Opiates. None makes you miserable and withdrawn. A tiny amount blocks pain. A moderate amount stops you from breathing. A LOT causes you to go into a hibernation-like state where your body doesn't suffer metabolic harm (my favorite quote from an ER doc was "The hypothermia saved his life"). Same breakdown, so you don't die as you come down/come out of hibernation.

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    $\begingroup$ I don't see how this satisfy the requirement of the lethal dose range 0 < X < lethal dose < Y. Can you make it clear? $\endgroup$ – L.Dutch - Reinstate Monica Jul 3 at 3:38
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This substance is a meat dish made from a poisonous animal.

In the wild, this animal exudes some poison over its skin in order to protect itself from predators, in addition to using it on its prey. As with most venomous species, the antidote is also obtained from the same animal; they need not to intoxicate themselves with its poisoned prey, so they naturally a generate an antitoxin as well, which is present inside their body.

This is a very appreciated dish by the noble families of this country, with its own protocol on how to eat it (just as most of their plates). As it happens, the guests will be eating a poisonous, outer piece, then an inner one afterwards which neutralizes it, and so on. This mixture is what provides its particular taste, so highly valued (actually the consequence that you start getting poisoned then immediately an antivenom, then some more poison and so on). The wine that is traditionally served with this dish also happens to cancel some of the poison activity.

It is also possible that some of the noblemen which use eat this animal on special feasts, have developed some immunity to this poison.

However, if someone from the group of the newly-arrived ambassador which was being honored with this summum of their culinary art didn't eat the whole piece served, or was picky about the parts they took… they could have ingested a lethal dose of the poison, but not enough antivenom to counter it.

That could cause quite a diplomatic issue.

Note that the aristocracy doesn't generally know the details about this, or that this dish involves taking a poison. From time to time, someone gets a bit ill after a banquet, but given the amounts of food served, that's not surprising.

Also keep in mind that this animal needs to be cooked right. If you overcook it, you would cancelling the outer venom, and it would lose its special flavor!

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    $\begingroup$ I don't see how this satisfy the requirement of the lethal dose range 0 < X < lethal dose < Y. Can you make it clear? $\endgroup$ – L.Dutch - Reinstate Monica Jul 3 at 3:36
  • $\begingroup$ Let's say 1 bite you're good, 2-3 mouthfuls you will die, you eat (almost) everything, you survive. Your preferred thresholds may vary, of course (or also take into account additional things like the age and weight of the victim). Maybe you can clarify where you had trouble in the explanation, @L.Dutch-ReinstateMonica? Was the backstory so colorful that it obscured the 'technical' reason? Note that a key point in the explanation is that this is meat, not a liquid, and the different parts from this animal are homogeneously poisonous. $\endgroup$ – Ángel Jul 3 at 21:35
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I have 2 solutions both at my knowledge do not exist IRL:

  • A solution may be that at high concentration the substance reacts with something in the blood like albumin or with itself, aggregating and becoming inoperant.

  • The second solution when it reaches a high enough concentration it triggers a chemical reaction from the body either immunologic or purely chemical leading to the inactivation of the substance.

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Your "poison" is electric charge delivered through clever use of pill coating materials.

The lethality of electric shock reaches a peak between 100-200mA, with shocks of lower current being less able to affect the heart, and shocks of higher current tending to clamp the heart completely, which it is often able to recover from.

Such a poison would require very sophisticated (definitely futuristic) and expensive manufacturing; it could involve putting an extremely high charge on conductive nanoparticles, then coating those particles in a layer of specialized material that is highly insulating and soluble at a slow and predictable rate. Once ingested, the particles would naturally disperse fairly evenly throughout the body (they would repel each other.)

With enough precise control of the coating thickness, the particles could all lose their insulation within a fraction of a second of each other. If the victim is grounded at that moment, they will receive a dose of current proportional to the number of particles ingested as all of the charge flees to the grounding point(s). (If not, they'll receive it when they're next grounded.) Depending on the number of particles ingested, you could have a dose < X causing unpleasant muscle spasms, a dose > Y causing burns and reversible cardiac arrest, or a dose between X and Y causing irreversible cardiac arrest and certain death.

Some interesting properties of this approach:

  • If the victim receives dose X/2 at 9pm, and dose X/2 and 10pm, the doses will not add up to a lethal dose, even if the release period is longer than an hour. Two separate releases will occur.
  • The basic version of the poison would be trivially detectable, but you could imagine more sophisticated and expensive versions which include masking of the charge with the opposite signed charge with a different coating.
  • If the victim is grounded through another person at the time of release, that person will receive an approximately equal dose. In this way, the "poison" can affect someone who was never dosed, but who is touching a dosed person.

The factor that requires mild hand-waving is the feasibility of near-simultaneous release of the soluble insulators. If you prefer, you could explain this with nanomachines, which would increase the technology level.

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