(dont use this in real life and dont sue me)
i have a special interest in poisons and doubt ill ever get to use it so i really, really hope this helps someone. take everything with a grain of salt, though, as some are from my memory, some are probably plain wrong, some might be misconceptions, but generally speaking these should be accurate.
i have listed some i think could be nice in writing:
CYANIDE is a fast acting, commonly used poison, killing within seconds or minutes depending on dose. cyanide works by blocking cells from using oxygen. you will expirience symptoms including dizziness, headache, cardiac arrest, low bp, seizures, shortnessof breath. id assume youd also have typical symptoms of low oxygen so pale and blueish extremeties and lips.
you can give supportive care to cyanide poisonings - includining charcoal, antidotes, and supplimentary oxygen. remember you can test for cyanide pretty easily.
TTX or pufferfish poison kills you via respitory failure as it paralysises your breathing muscles. it needs an extremely low dose to kill, and only has supportive care. ttx does not have an antidote, its just waited out when possible and has assisted support - mostly mechanical ventilation.
THALLIUM is naturally occuring in the earth, has no taste, smell or colour but is slow. it requires a fairly large dose, is hard to find, but is very slow, planned and hard to find in a corpse unless thallium is specifically a suspect. it acts similiar to food poisoning and diseases similiar to it. it takes up to a few days to kill and will ramp up the 'illness' during it. It can also develop neurological symptoms, but near all of its symptoms mimic other disorders. There IS an antidote - Prussian blue.
BOTULINUM or botox - which causes muscle paralysis. it kills because it paralyses your muscles, and only including murder attempts/accidents/ect and not medical uses or cosmetic uses, it tends to specifically kill via respitory failure as it causes respitory paralysis and seizes someone. youll present low oxygen symptoms and die within minutes. it does not always kill, though. botulinum poisoning is called botulism.
ARSENIC can cause nausea, vomiting, diarrhea, severe dehydration, ect. it can cause in low, consistent doses hyper- or hypopigmentation which is why historically it was used to lighten someones skin. it increases risk for several cancers, and supposedly with chronic exposure can actually cause diseases such as diabetes, heart disease, cognitive impairment, .... It can be ingested but most toxic is inhaling it. apparently, heated arsenic smells like garlic. the cure is chelation therapy, removing the source of arsenic, and most likely also activatef charcoal like most poisons.
RICIN is incredibly deadly. the amount of a grain of salt can be fatal. it has three ways to enter your body - inhallation, ingestion, and injection. inhalation causes respitory failure - fluid in the lungs, fever, hypersweating, coughing, shortness of breath, ect. Ingestion causes organ failure usually after several days - having liver, spleen and kidney failure. symptoms including hallucinations, dehydration, cramps, abdominal bleeding, vomiting, bloody urine. Injection causes internal bleeding and multiple organ failure swelling and pain of the muscles and lymph nodes.
these are common poisons for a good reason. theyre known, generally accessible in most universes, and are fun to write. they are, however, misused a lot. please. there are SO. FEW. POISONS. that kill during one meal.
more obscure poisonings;
THC which is found in cannabis can actually be fatal. not in the amount you smoke or eat in edibles, but thc as a compound can actually kill within 30 minutes depending on dose. its rare and doesnt really have many sources in writing because of that, but as its mind altering it would likely kill via psychosis or in children, ataxia. its symptoms are extreme versions of being high - high heartbeat, weird breathing, hallucinations, ect. THC poisoning causes psychosis and there ARE recorded deaths. id very much assume youd die from self injury or respitory failure.
TREE OF DEATH (in spanish), or MANCHINEEL. its a tree where every single part is poisonous - from the bark and tar to the leaves and fruit. they tend to be marked with red X's in some countrues so you avoid it. its sap causes bulbous dermatitis, its tree causes blisters, burning the tree causes the smoke to give you ocular injuries, and its fruit can be fatal via severe gastrientiritis and bleeding, shock, bacterial superinfection or lack of oxygen.
BTX or BATRACHOTOXIN is an extreme cardiotoxin and neurotoxin where it causes affects on the heart muscles,prevents sodium channels from closing, paralysis, convulsions, cardiac arrest, twitching, numbness. it kills within minutes with sufficient dose. one way ive heard it kills you is it makes you convulse so much you block your own airway. no treatment, just supportive care and hope.
enjoy and write responsibly :D
edit; added thc clarification