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I’m writing a (fictional) story, and at one point the main character is temporarily paralyzed due to poison. Does anyone know of a poison that would, if taken in a very small amount or if it just touched one’s lips would paralyze you for a period of time, but if ingested in higher amounts (say, a swallow or two) would kill you? I’ve considered aconite, but can’t find any good sources on if that would actually work for the purpose described above.

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    $\begingroup$ Nice try, Vlad Putin, but it will not work here $\endgroup$ – Bald Bear Nov 24 '18 at 5:25
  • $\begingroup$ Good to have these things clarified at the start $\endgroup$ – nzaman Nov 24 '18 at 6:40
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    $\begingroup$ that will be true of most paralytics, once they paralyze the diaphragm your done. $\endgroup$ – John Nov 25 '18 at 18:55
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Botulinum toxin

It prevents the release of the neurotransmitter acetylcholine from axon endings at the neuromuscular junction and thus causes flaccid paralysis.

The toxin is also used commercially in medicine, cosmetics and research.

You have an example of this with all those celebrities who use "botox" to flatten their wrinkles (at the cost of losing facial expressivity due to the paralized muscles)

Botulinum is the most acutely lethal toxin known, with an estimated human median lethal dose (LD50) of 1.3–2.1 ng/kg intravenously or intramuscularly and 10–13 ng/kg when inhaled.

It can be easily found in non properly prepared canned food

as food canning was approaching a billion-dollar-a-year industry, botulism was becoming a public health hazard.

Since preparing a proper dose of botulin can be difficult, you can also use

Fugu:

Fugu can be lethally poisonous due to its tetrodotoxin; therefore, it must be carefully prepared to remove toxic parts and to avoid contaminating the meat.

Fugu contains lethal amounts of the poison tetrodotoxin in its inner organs, especially the liver, the ovaries, eyes, and skin. The poison, a sodium channel blocker, paralyzes the muscles while the victim stays fully conscious. The standard treatment is to support the respiratory and circulatory systems until the poison is metabolized and excreted by the victim's body.

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  • $\begingroup$ I can't work out if botulin causes non-lethal paralysis from your answer. If the respiratory & circulatory systems need supporting to survive tetrodotoxin, this suggest the paralysis it causes may be lethal. Both examples could be improved with more information about their non-lethal and lethal dosages. Otherwise, very good. Nice choices. $\endgroup$ – a4android Nov 24 '18 at 9:38
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    $\begingroup$ @a4android, many celebs use botulin, or botox, to paralize facial muscles and flatten wrinkles. About fugu, I have read somewhere that the skill of the cook is in leaving enough toxin in the prepared fish to "tingle" one's mouth but not paralyze them. The same source said there are people actually using self prepared fugu to fall into a self induced paralysis, which they apparently enjoy. Considered the topic I don't feel comfortable in indicating dosage (IANAD). $\endgroup$ – L.Dutch - Reinstate Monica Nov 24 '18 at 9:43
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Most neurotoxins will produce results like this, but you would have to be very careful because the difference between a lethal dose and a non-lethal one is generally very small. They also have a reasonably high risk of causing lasting damage if not used carefully.

Examples of easily available neurotoxins that would probably work include:

  • Botulinum toxin. Already mentioned in one of the other answers, so I wont' cover the specifics here. This is a really risky one though, as the median lethal dose is about 2 thousandths of a microgram per kilogram of body weight.
  • Tetanospasmin. A toxin produced by the vegetative cells of Clostridium Tetani, the bacteria that causes tetanus. There's a slightly bigger margin of safety here than with botulinum toxin (the difference in LD50 is about 0.5 thousandths of a microgram).
  • Saxitoxin. This is the primary toxin responsible for the toxicity of red tides. It has well characterized behavior in living things, is pretty easy to harvest, and can easily be added to a dish of food (it can often survive cooking), so it's probably your best bet. Median lethal dose for adult humans is about 5.7 micrograms per kilogram (roughly a single grain of sand for a normal adult male), so you've got a much wider margin of safety here than for the first two.
  • Tetrodotoxin. This was also mentioned in one of the other answers, though by reference to one of it's sources (fugu). It can be found in a wide variety of marine life, so it's pretty easy to find. It's not quite as dangerous as the first two, and pretty reliably produces paralysis (most marine life that produces it relies on this fact for defense or to hunt). Median lethal dose for humans is similar to but higher than saxitoxin (I couldn't find conclusive information on an exact value).
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