I have an alien species of roughly earthlike anatomy and biochemistry (so they can consume earth food, and humans would be able to digest their foods as well)

These species have a trait that allows them to consume poisons and toxins and not get ill or die. The current justification is that it's an evolved trait since their homeworld has an ecosystem where a large percentage of animals and plants are either venomous or poisonous, so the aliens' digestive tract got modified in some way to neutralize, isolate, or break apart those harmful substances. (story-wise this is needed for the aliens to not be picky eaters, or die of starvation or poisoning when they got stuck on Earth)

So how can they digest poisons effectively?

Bonus points for a solution that makes them able to take care of organic poisons and toxins but at the same time doesn't offer much protection from some synthetic toxic substances. (Initially, this bit was inserted to "balance out" their abilities as to avoid MarySue-ism, but I won't mind removing it if it isn't compatible with the main goal)

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    $\begingroup$ Honey badgers, in addition to their temper, are notorious for their resistance to toxins, not only sometimes eating poisonous things with little adverse effects, but actively getting bitten by venomous snakes and shrugging it off. In fact: our planet has a ton of creatures that can not only eat poisonous things, but also incorporate that poison into themselves through their diet, in turn becoming poisonous (seen in how many normally poisonous species are harmless in captivity due to their diet not including the toxic things they'll usuall eat in nature). $\endgroup$ Sep 29, 2022 at 14:34
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    $\begingroup$ Poison is a relative thing. A race of sapient dogs might as well ask how humans can digest onions, for example. $\endgroup$
    – DKNguyen
    Sep 29, 2022 at 22:59
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    $\begingroup$ Which toxins, everything is toxic in the right dose. humans digest a variety of toxins better than any other animals, particularly maillard compounds. $\endgroup$
    – John
    Sep 29, 2022 at 22:59
  • $\begingroup$ A venom that is fatal when injected into muscle or blood (e.g., a snake bite) might be completely harmless when ingested and broken down by stomach enzymes. $\endgroup$
    – doneal24
    Sep 30, 2022 at 18:00
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    $\begingroup$ "Poisons and toxins" only has real meaning when applied to specific species. What is poisonous (or venomous) to one species may not be to another. For instance, oxygen is highly reactive and caustic and was almost certainly highly poisonous to the anerobic life that preceded the Great Oxygenation Event (which is probably why some specifies started generating it). Likewise many venomous animals are relatively immune to their own toxins. $\endgroup$ Sep 30, 2022 at 20:29

7 Answers 7


Poisons and toxins are just biological molecules, even in our world some species can digest what is toxic for others. As long as there are the needed enzymes to break them down or the needed molecules to use them, they will have no effect.

Just to give you some examples:

  • do you remember the Rambo's quote?

he has been trained to feed on thing which would make a goat puke

  • caffeine is developed by plants as defense against parasites, we drink coffee fondly
  • we eat chocolate, which is toxic for dogs
  • when fishes feed on toxic algae, they become toxic for us, despite not being killed by it
  • we breathe oxygen, which is toxic for all anaerobic life forms. Which in turn can breathe H2S which is toxic for us
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    $\begingroup$ Upvote for honorific "the" before Rambo. I aspire to be The Rambo. Part of fulfilling that aspiration is tracking down the goat puke thing list. $\endgroup$
    – Willk
    Sep 29, 2022 at 22:39
  • $\begingroup$ I'd add that caffeine is poisonous to us still, but the chances that humans get to 80mg/L-100mg/L of caffeine in their blood is really, really rare, short of not taking only coffee and also taking caffeine tablets. $\endgroup$ Sep 29, 2022 at 22:55
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    $\begingroup$ @AlexanderThe1st Yes, but as is often said, “the difference between poison and medicine is in the dose.” Most plants produce far too little caffeine to reasonably poison a human, though one exception may be holly — specifically Ilex vomitoria — which has the highest caffeine of any North American plant and has a history of use in some indigenous cultures. $\endgroup$
    – Greenstick
    Sep 30, 2022 at 0:07
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    $\begingroup$ We recreationally consume alcohol, THC, MDMA, LSD that all try to royally screw with our brain chemistry. If those aren't close enough to biological poisons then recall that some of us lick poison dart frogs for kicks. $\endgroup$
    – dbmag9
    Sep 30, 2022 at 14:02
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    $\begingroup$ Your 3rd point is an important one. Many earth animals, especially insects, eat specific plants to make themselves toxic to predators. $\endgroup$
    – Theodore
    Sep 30, 2022 at 20:29

This is a non-problem.

Every day horses and rabbits around the world eat all manner of green leafy things that would kill you if you only managed to swallow enough of them. possible citation

On the other hand many of the vegetables that go into a tasty vegetarian lasagne will quickly kill your housecat. citation

However we do not put big warning stickers on houseplants, or on the carrots and peppers in the supermarket. This is because poisonous is a relative term. Everything is poisonous to everyone. This is to stop it being eaten. . . except by the animals that can eat it. It is not poisonous to those guys.

It makes perfect sense that an alien that eats loads of different things is not killed by those things.

  • $\begingroup$ Upvote for societally conscious scheme to put tasty vegetarian stickers on housecats. Although the alien is killed by those things - it only ate those loads one time when it was drunk. Maybe a sticker on that drunk dead alien too, as a warning. Because it wasn't dead, just very drunk and now also gassy. The stickers should say that. Not the ones for the cat though because that would not make sense. $\endgroup$
    – Willk
    Sep 30, 2022 at 0:20
  • $\begingroup$ Household items that will kill a housecat include garlic, onions, chocolate, vitamin D3, and house plants. Only the first two tend to be included in lasagne. Locoweed is poisonous to humans in very small quantities but is relatively palatable to livestock, including horses. $\endgroup$
    – doneal24
    Sep 30, 2022 at 18:11
  • $\begingroup$ @doneal24 I suspect the pasta is also bad for the cat to eat. $\endgroup$
    – Daron
    Sep 30, 2022 at 19:40
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    $\begingroup$ Are you implying that Garfield is an alien cat with his known love of lasagna? $\endgroup$ Oct 1, 2022 at 4:24
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    $\begingroup$ @MarkRansom Garfield is now 46 years old but shows no signs of aging. Being an alien is one possible explanation for that. $\endgroup$
    – doneal24
    Oct 1, 2022 at 13:58

Honey badgers, in addition to their temper, are notorious for their resistance to toxins, not only sometimes eating poisonous things with little adverse effects, but actively getting bitten by venomous snakes and shrugging it off. In fact: our planet has a ton of creatures that can not only eat poisonous things, but also incorporate that poison into themselves through their diet, in turn becoming poisonous (seen in how many normally poisonous species, such as the infamous dart frogs, are harmless in captivity due to their diet not including the poisonous things they'll usuall eat in nature).

At the end of the day, poisons and venoms are simply proteins that happen to mess us up. If you have the biological means to render those proteins inactive and has the right enzymes to break them down, you're more than fine.

Another fun fact: many snake venom can kill us, but if you were willing to do it, you could become immune to certain venoms/poisons by regularly injecting yourself with small enough doses in order to allow your body to build an immunity, a more modern example being Steve Ludwin (I'll note this however: DO NOT do this without proper knowledge, supervision and/or proper access to antivenoms and a doctor, snake venom is not a joke, even if you're immune to it).

Overall, it's really a matter of natural selection. If your world has a large amount of venomous/poisonous animals and plants, then animals without the ability to handle said venoms and/or poisons have less available options and are much more reliant on a smaller number of species to survive. Meanwhile creatures that can handle the venom and/or poisons (like being able to digest a poisonous plant or being able to handle envenomation from venomous prey) can eat from more sources, potentially grow bigger and stronger thanks to a more plentiful diet and outcompete non-resistant competitors, getting to share the mutations that allowed them to be resistant to the next generations.


Afaik (and I'm not an expert on this), poisons and toxins are just substances that are similar enough to the ones we need or are accustomed to handle and for which we have receptors upon which they can dock on and transmit their information, effect or just stick to, while they produce a side effect that is very different and often harmful compared to the effect that the body would expect.

So let's you say live in a cave and once a day someone comes there flashes a light to grab your attention and drops a piece of food. You open your mouth and eat it. That process gets so ingrained in your biology that a flashing light directly triggers opening of the mouth. Now consider someone approaches the cave with a stroboscope light and your jaw would break because of all the opening and closing of your mouth. Something like that but in the realm of biochemistry. So for this person in the cave light in the wrong dosage (in that case too fast) would be a toxin.

Likewise you could imagine a robot species who wouldn't so much "digest" food as they would just BURN all organic matter and transfer the heat into a turbine generating energy, so they can "eat" roughly all that a human can eat + toxins and poisons (because there are no receptors just a box with heat and fire that drops out burned matter), but for example if you feed them a gooey substance which is fire resistant this will clog it's "stomach" and prevent them from "eating", thus not generating energy and thus would "kill" them. So despite not being toxic on the biochemistry front, this would for example be a robot-toxin. Hope this helps.


Snake or spider venoms can typically be drank by HEALTHY humans with no ill effects. This is because "venoms" describe substances that are lethal if introduced to the blood stream. By the time this happens in the human digestive tract, powerful stomach acids will have broken down the venom into amino acids that the body can then use to build useful proteins (For a scientific analogy, an Amino Acid is to a Proteins what a Lego brick is to a Lego Set... only Amino Acids aren't going to hurt your feet in the middle of the night.) Venom is nothing more than a specialized proteins created by the snake/spider/whatever. Venoms are only posionous if they enter the blood stream. If consumed by a healthy human, it's no more fatal than drinking a proteins shake.

However, there is a catch. If a human has a stomach ulcer (or any other kind of ulcer in the digestive track) the Venom can enter the blood stream and do that thing they do best... kill someone. So while we can digest snake venom if we consume it orally, it's generally best if we avoid drinking venoms. Additionally, certain venoms contain Necrotoxins which will kill cells (and human skin is made of those) and could get into the blood stream getting into blood that is exposed by dead skin, so you can't just drink any type of venom... you got to know about the animal that made it.

Additionally, there are animals that are non-venomous but are toxic. Venom is specifically used as an offensive mechanism and typically has to be injected into the blood stream. Toxins refer to chemicals that are organic poisons... and defensive toxins tend to be fatal if consumed to discourage would be predators from eating the plant or animal that consumes them.

As said by other people, humans can consume certain substances used as defensive Toxins because humans evolved from omnivorous hunter-gather forages. As far as animals go, we are not picky eaters and can consume a wider variety of foods than most animals. We're one of the few mammals in which lactose intolerance in adults is an exception, not a rule, and even rarer we routinely consume the milk of other mammals, not just humans. Our stomachs are evolved to get nutrients from a wide array of food stuffs, compared to other animals that are almost exclusively herbivores or carnivores and cannot digest meats or plants specifically.

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    $\begingroup$ DISCLAIMER: Please do not try this at home. $\endgroup$
    – Tom
    Sep 30, 2022 at 3:11
  • $\begingroup$ @Tom How many spiders would I have to milk to get a shot glass of venom? Is it realistic to try this at home, especially since this is not a TicTok challenge. :) $\endgroup$
    – doneal24
    Sep 30, 2022 at 18:13
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    $\begingroup$ @Tom is right folks. Although I listed reasons in my answers, can't stress this enough. This is an Ian Malcolm problem right here: Just because you CAN do this, it does not mean you SHOULD do this. $\endgroup$
    – hszmv
    Sep 30, 2022 at 18:16
  • $\begingroup$ @doneal24 Depends on the size of the spider. $\endgroup$
    – hszmv
    Sep 30, 2022 at 18:17

It’s not only believable that they could safely eat things humans could not, it’s almost expected.

Put simply, toxicity is a function of biochemistry and dose.

Consider for a moment why penicillin is useful as a medication. It’s lethally toxic to many bacteria in doses that are not generally enough to have any measurable impact on mammalian cells (even most of the side effects are actually a result of it disrupting a person’s natural gut bacteria). Many (but not all) other antibiotics are relatively similar. This arises from the fact that bacterial biochemistries are different enough from human biochemistry that there is limited overlap in toxicity of many toxins.

In contrast though, many antifungal and antiprotozoal medications (for example, amphotericin B or quinine) are actually mildly toxic to humans, and require much more careful dosing as a result, because fungi and protozoans have much closer biochemistries to those of humans.

Even some ‘regular’ medications that aren’t trying to kill specific things have relatively narrow gaps between being safe and medically useful and being lethally toxic. Digoxin, atropine, and warfarin are all prime examples of this (all three are considered essential medications by the WHO, all three are also nastily toxic if you have even just a bit too much).

You can find a number of other examples as well:

  • Most felid and canid species cannot safely eat many species of Allium, though humans use Allium species regularly as food (onions, leeks, shallots) or seasonings (garlic, chives) with essentially no ill effects.
  • Chili peppers are ‘spicy hot’ to pretty much all mammals, but not to most birds.
  • Atropa belladonna (commonly known as deadly nightshade) is nastily toxic to humans and most other mammals, but a number of bovid and leporid species have no observable issues eating it.
  • Horseradish is perfectly safe for humans to eat, but potentially lethally toxic to horses (which is, ironically, where it’s name comes from).

Veterinarians can probably easily list dozens of other examples of things that are toxic to certain animals, but not humans.

But it inherently won’t work for everything.

Barring drastically different biochemistries, some things will still be toxic to both this species and to humans.

For example, most metals and other ‘pure’ elements are toxic regardless of species because of limitations in terrestrial biochemistry. In particular, most cells cannot differentiate very well between different ions with the same ionization state.

Arsenic poisoning is a prime example of this. Arsenic ends up as arsenate ions in the body, which look to your cells like they’re no different from phosphate ions. As a result, those arsenate ions end up getting substituted for phosphate ions when the cells are producing adenosine triphosphate (ATP). But the arsenate groups don’t bind anywhere near as strongly as the phosphate groups do, so the adenosine diphosphate arsenate that gets produced in place of ATP just kind of falls apart, wasting the energy that it was supposed to be storing.

This ultimately means that pretty much anything that uses ATP and produces it in a similar way to humans is affected by arsenic poisoning.

You can find similar (and usually even more broad spectrum) cases for most other elements known to be toxic. A lot of these are still likely to be toxic to your aliens given that they have a close enough biochemistry that they can share food with humans.


Even the common deer can eat many mushrooms that are toxic to humans. It's likely due to the differences in their gut microbiome which can break down the toxins much more efficiently than the average human's before they can pass into the body.

Some research into composting (Humanure Handbook - Compost Miracles) can reveal some pretty amazing things that can be accomplished by microscopic life: degrading toxins like chemicals, petroleum fuels, PCBs, insecticides, herbicides and even explosives, into simpler benign organic molecules. Of course a digestive system is not a compost pile (it's anaerobic-based for one thing), but it does show how an organic system on Earth does things you're looking for, albeit much more slowly than typical animal digestion.

Tying the aliens' toxin resistances to their gut microbiomes could open them up to some interesting weaknesses based around that too. The synthetic toxic substance you mention could be something the microbiome couldn't consume, or maybe even be a genetically designed bacterium or protozoa that results in a version of Traveller's diarrhea that flushes enough of their guts' protective microbiome in a way that it renders them temporarily vulnerable to toxins they normally would be able to consume safely.


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