There are some general principles to bear in mind when considering using poisons in a plot:
What's the fatal dose?
Crucially, the dose makes the poison - a small amount of something potentially harmful may have little effect but a larger dose could be fatal. Cyanide, for instance, is released from various fruit pips and stones but not in dangerous quantities.
Individuals vary in their response to poisons. Some may be relatively unharmed by a dose which could produce severe symptoms in someone else.
It's possible to build up a tolerance to some toxic materials - contrast, for instance, the effects of three double whiskies on a hardened drinker with those on a teetotaller or the effects of a cigarette on a non-smoker. Dorothy L. Sayers used this principle with arsenic in Strong Poison and some people who have been bitten repeatedly by poisonous snakes can become immune to their venom. It doesn't always work as poisons are handled in different ways by the body so don't try this at home. The opposite phenomenon, sensitisation, occurs when an initial exposure to a substance makes a vulnerable individual more sensitive to its effects the second time it is encountered - this happens with some industrial chemicals
Getting to work
Poisons can enter the body via ingestion (eating or drinking), inhalation, injection, absorption through the skin, absorption through mucous membranes (lining of the mouth, nose, vagina) or rectally. The effects and speed of action depend on the route of administration and how dilute the poison is. A full stomach takes longer to absorb a poison than an empty one.
Poisons take time to work. Once taken in, the substance has to reach the target organ on which it acts - for instance the brain, heart or nervous system. The classic scene where someone sips from a poisoned cocktail and immediately clutches their throat and falls down dead does not happen - it would take several minutes, sometimes longer.
Acute or chronic?
In acute poisoning symptoms appear relatively rapidly from a single dose. Chronic poisoning results from repeated exposure over time and symptoms may develop slowly. Something which is rapidly eliminated from the body (if it has not proved fatal) is unlikely to lead to chronic poisoning. Substances which are retained by the body (e.g. metals and metalloids such as thallium, mercury and arsenic) can build up to dangerous levels on repeated exposure (see, for instance, Agatha Christie's The Pale Horse where repeated doses of thallium led to death).
Kill you later
Some poisons have a delayed effect with death occurring some time after ingesting a fatal dose. An overdose of paracetamol can take many hours to kill as it slowly, and painfully, destroys the liver. Similarly, people eating a lethal quantity of the poisonous fungi Death Cap or Destroying Angel appear to recover when the initial symptoms of nausea disappear. They die later, inevitably, as the liver is wrecked.
Once a substance is absorbed by the body various mechanisms operate to remove it. It may be excreted in urine, exhaled if it is volatile (as are many solvents), or broken down in the liver or other tissues. Swallowed irritant poisons, such as arsenic and antimony, may be vomited out but often not before a dangerous amount has been absorbed. Some chemicals may be stored in fatty tissues (e.g. pesticides like DDT), deposited in bones (metals like lead) or incorporated in hair (metals and various drugs). In most cases it is the amount of a poison/drug in the bloodstream which is crucial - the peak concentration reached and how long significant levels remain determine the extent and duration of effects.
A key property of many substances which can affect the body is the half-life. This is the time taken for the concentration of the substance, e.g. in blood, to decline to half its original value. After the next half life has passed the concentration has dropped by a half again (this pattern occurs because the rate of elimination of the substance depends on the concentration present). So, if you start with 40mg of something per 100ml of blood, after one half life you will have 20mg per 100ml; after two half lives you will have 10mg per 100ml, after three half lives, 5mg per 100ml and so on. Half-life is important because it determines how quickly the effects of a drug will wear off (assuming a fatal dose has not been taken) and how long residues will remain in the system for subsequent detection. Ketamine, for instance, has a half-life of 2.5-3 hours. When it is used as an anaesthetic patients wake up after 5-10 minutes although it can take considerably longer for feelings of wooziness to disappear. Flunitrazepam (Rohypnol), however, has a half-life in the blood of 11-25 hours and its effects last longer, in part because it is metabolised to another biologically active compound which has an even longer half-life.
Not all substances show this pattern. The rate of elimination of ethanol (drinking alcohol), for instance, is fairly constant. Also, the half-life of a substance will depend on the exposed individual's ability to eliminate or metabolise it.
Is it detectable?
There is no such thing as an undetectable poison - if an analyst knows what to look for it will be found. There are thousands of different potentially toxic substances so not all will be looked at routinely in the case of a suspicious death. Clinical signs pre-mortem and indications at autopsy, however, can provide clues. Sometimes metabolites (breakdown products formed in the body) are looked for rather that the poison itself. The poison which killed Georgi Markov (ricin) was not actually found in his body or in the pellet fired into his leg by his assassin. Its identity was inferred from his signs and symptoms plus the fact that it had to be one of a few, particularly lethal, substances since only a tiny amount of material could have entered his body.
Detecting residues of short half-life drugs can be difficult. If GHB is suspected as a date-rape drug a urine sample must be taken within four hours of the drug being ingested although residues can be detected in hair months later. Post-mortem the situation is more complicated as GHB is produced naturally, in small amounts, in the body. The substance may build up after death, although not to levels indicating that a large dose of GHB had been taken. Traces of Rohypnol, and its metabolite, can be found in urine for days afterwards if the correct analytical technique is used. Ketamine can be detected in urine 3-5 days after last use by commercial drug testing kits and its metabolites can be found up to 10 days after ingestion if a more sophisticated technique is used.
Disguising a death from poisoning as natural causes can sometimes work (see, for instance, Harold Shipman's infamous use of opiates and cases of insulin overdoses in hospitals) but if suspicions are raised, further investigations will often reveal the cause.
Where can I get some?
Many traditional poisons are not readily available in the UK. Some are now banned completely and others may only be sold by retailers with a licence who must report any suspicious purchasers to the police. The more dangerous agricultural pesticides have now been discontinued although there may still be old stocks of these - and other - poisons lurking in sheds, garages and outbuildings.
Plants can be a source of poisons for a potential murderer but getting someone to eat enough to prove fatal can be difficult (with a few exceptions which I won't go into). Often the plants are unpleasant to eat. Extracting the poison would give a surer means of committing murder but this requires a little skill as well as access to solvents, basic apparatus etc. Digoxin, atropine and strychnine are relatively easy to extract although the careless operator could well end up as the victim rather than the perpetrator. Trying to extract pure ricin or abrin from plant material is tricky, likely to prove dangerous to the unskilled operator, and also an offence under anti-terrorism legislation. Amy Stewart's book Wicked Plants contains some interesting information on poisonous plants, albeit with a North American focus.
Poisons unavailable in the UK have been imported - David Lytton ("Neil Dovestone") brought back the strychnine with which he committed suicide on Saddleworth Moor in 2015 from Pakistan and a plant containing aconite was brought in from India by a woman who used it to poison her ex-lover in 2009 (see http://news.bbc.co.uk/1/hi/england/london/8490795.stm).
The internet is a potential source of toxic chemicals unavailable in the UK although using it leaves a trail and there could be problems getting material through customs. The same applies to the dark web where, we are led to believe, everything is available for a price.
Cyanide is, chemically, an ion - a carbon atom connected to a nitrogen atom and bearing a negative charge. It is normally encountered as potassium or sodium cyanide where the cyanide ion is combined with either sodium or potassium. These compounds are colourless and tasteless white crystals which dissolve easily in water. If the cyanide ion is combined with hydrogen the resulting compound is hydrogen cyanide, a gas which dissolves in water to form prussic acid and which smells of bitter almonds (though not everyone can smell it). Adding sodium or potassium cyanides to an acid releases hydrogen cyanide gas, which is the basis of the US gas chamber. Stomach acid will release hydrogen cyanide from the potassium or sodium compounds which is why the breath of someone poisoned with a cyanide compound may smell of bitter almonds. It is also the reason why a post mortem on someone thought to have ingested cyanide is carried out under special conditions to prevent the pathologist succumbing to the effects of the poison.
Death from cyanide poisoning can occur within a minute or two if it is injected or inhaled. Swallowed cyanide can take up to a quarter of an hour to kill, especially if taken on a full stomach, as it has to be absorbed before it can reach the tissues. There are treatments but they need to be deployed very quickly if they are to work. Cyanide victims do not look blue: they may look flushed red or pink although this is not always the case. In the UK cyanide compounds are not generally available to the public although they have a number of industrial uses and can be supplied for specific authorised purposes.
Cyanide works by preventing the body from using oxygen - hence the pink colour mentioned above, caused by bright red fully oxygenated blood which cannot pass on its oxygen to vital processes within the cells.
Carbon monoxide, a highly toxic gas, was an ingredient of the coal gas which used to be supplied to homes and industry, hence putting your head in an unlit gas oven with the gas on would rapidly lead to unconsciousness, coma and death. Coal gas has been superseded by natural gas which is predominantly methane so this means of suicide (or murder) is no longer effective. Methane is non-toxic although it can suffocate by excluding air from the oven - the victim would need to be unconscious beforehand.
Carbon monoxide is also released by motor vehicle exhausts. The catalytic converters which are now fitted to cars destroy much of this once they are warmed up but a cold car, just started up, would produce considerably more. Running a car in a closed garage (or connecting a hosepipe between the exhaust and the interior) would still prove fatal - it just takes longer to do so. Given the unpleasant smell of vehicle exhausts a murder victim would have to be incapacitated in some way for the gas to take effect - which would leave traces.
Several fatalities have occurred as a result of badly maintained gas appliances or blocked flues. In these cases the gas does not burn properly, giving rise to high levels of carbon monoxide instead of carbon dioxide. Deliberate tampering to produce this result has been used in novels, including Rachel Abbott's Sleep Tight.
Carbon monoxide poisons by combining with haemoglobin, the red pigment in the blood. This prevents oxygen combining with haemoglobin so the blood cannot carry oxygen to the tissues. Because it binds so tightly to the haemoglobin it only takes a low concentration of carbon monoxide in the air to produce symptoms of headache, dizziness, confusion, sleepiness and, eventually, death.
Nicotine is a clear, colourless liquid which goes yellow-brown on exposure to light and air. It is not responsible for the yellow colour on smokers' fingers or wallpaper and ceilings where people smoke - this comes from tobacco tars (Stuart Macbride got this right in A Dark so Deadly: many people don't). It would certainly not turn a corpse yellow. Nicotine is highly toxic by ingestion or inhalation and is also absorbed through the skin. It has been used as an insecticide and boiling up cigarette ends to make nicotine tea for use in the garden is a traditional means of extracting this substance. This would be revolting to drink, however, so a chemical process for purifying the nicotine would be needed if this was to be used for murder. Note that regular smokers have a tolerance to nicotine and would require a larger dose to kill them.
In 2012 the unexpected death in Surrey of Russian exile Alexander Perepilichny prompted suspicions that he had been poisoned by Russian assassins because of his alleged whistleblowing on organised crime in that country (http://www.dailymail.co.uk/news/article-3088717). Traces of the highly toxic plant Gelsemium elegans were reported to have been found in his stomach although the cause of death was diagnosed by medical experts as heart failure during initial inquest testimonies. The inquest was adjourned in 2016, because of the involvement of the security services (see http://www.reuters.com/article/us-britain-russia-perepilichny-idUSKCN11C21L) Eventually, the coroner determined that Perepilichny died of natural causes but suspicions remain. His widow has denied that had received threats (but see, perhaps, Rice-Davies in R v Ward, 1963??)
There is no doubt that plants of the genus Gelsemium are highly toxic. In the US Gelsemium sempervirens, known as Carolina, or Yellow, Jessamine, has caused poisonings and it is reported, perhaps anecdotally, that some cases resulted from eating poisoned honey gathered by bees from its flowers. The nectar is known to be toxic. The more poisonous Gelsemium elegans is known as Heartbreak Grass and grows in Asia where it is said to have been used for many years by Russian and Chinese assassins. For more details see https://awkwardbotany.com/2016/09/14 and https://www.ncbi.nlm.nih/gov/pmc/articles/PMC3249920/.
Gelsemium also made an appearance in the 2017 UI.S. series of House of Cards where it was used to poison someone threatening to expose a corrupt regime.
I've come across several instances where a pathologist or police officer has said "traces of benzodiazepine were found". This is too vague as the term covers a whole range of drugs from diazepam (Valium) to flunitrazepam (Rohypnol). They have different effects, times taken to act and durations of action. Furthermore the times taken for residues to disappear also vary. The type of benzodiazepine should always be specified.
There have been numerous attempts to kill people with weedkillers (herbicides) but, fortunately, these have not proved very successful in recent years. Apart from arsenic, which was used in the distant past, the most poisonous weedkiller in common use was paraquat which is now banned in Europe because of its toxicity and threat to the environment. There have been tragic accidents when people have accidentally swallowed paraquat (see, for instance,http://www.dailymail.co.uk/news/article-1337101) and a few attempted murders such as the Chinese poisoned pants case https://www.theguardian.com/science/blog/2016/may/27. There is still a misconception, however, that all weedkillers are lethal. I recall one case where a woman tried to poison her husband with what she presumably thought was paraquat - in fact it was glyphosate, a much less toxic chemical. He got a stomach ache: she got eighteen months for administering a noxious substance.
The nerve agents sarin and VX have been used to kill people (Tokyo underground and Kuala Lumpur airport respectively) but these would not be feasible as poisons in a crime novel unless the murderer is backed by a state or a well-funded terrorist group with extensive chemical knowledge and skill. They cannot be made by kitchen-sink chemistry without killing the manufacturer.
Sarin has been used by military forces, with ghastly consequences.
Biological agents have been used in novels, from Agatha Christie's villains using anthrax bacteria (in Cards on the Table) and pus from an infected cat's ear (in Murder is Easy) to Peter James's killer using poisonous snakes in Love You Dead and Paul Finch's attacker using spiders and a scorpion in Hunted.
The lethality of some venomous animals is often over-rated. Most spiders, including the notorious Black Widow, do not inflict bites fatal to adults although they can be painful enough to distract a driver and cause a car crash (see Hunted above). Children and adults suffering from allergies or other vulnerabilities are more at risk. The same goes for scorpions. The large Emperor Scorpion, sold as a pet, may give a painful sting but is not dangerous, by contrast with the smaller Fat Tail and Death Stalker scorpions found in Middle Eastern deserts. Amy Stewart's book Wicked Bugs provides much detail about harmful invertebrates and their effects on humans. Venom, by Ronald Jenner and Eivind Undheim (published by the Natural History Museum) is a comprehensive guide to venoms throughout the animal kingdom.
Acquiring dangerous animals is not that easy in the UK (the bite of our only venomous snake, the adder, is very rarely fatal to adults) while attempts to cultivate infectious bacteria, such as anthrax (Bacillus anthracis), without specialist equipment and training would be highly dangerous.
Radioactive material can be used to poison people, as happened to the former FSB officer Alexander Litvinenko in 2006. Russian agents tracked him to London and added the radioisotope polonium-210 to his tea. He died from radiation sickness three weeks later. A radioisotope is an unstable form of an element which emits radiation as it decays. Depending on the type and intensity of the radiation, it may be safe to handle with minimal protection or dangerous to approach without proper shielding. High doses of radiation will kill, although not instantly, by smashing up cells and tissues while lower doses can lead to cancer as mutations are caused in DNA. The polonium-210 used to kill Litvinenko undoubtedly came from a state nuclear facility and would not be available to individuals.
The rate at which radiation is emitted from a radioisotope declines over time and is determined by the isotope's half-life (see above, under Hanging around, for an explanation). Half-lives range from tiny fractions of a second for some artificial elements to tens of thousands of years.
Attempts to poison someone with industrial radioisotopes would be difficult and would expose the perpetrator to, possible fatal, radiation levels. As with old poisons, however, there could be odd examples of radioactive products lurking in sheds - after all, radium was once sold in products allegedly beneficial to health! Catherine Caufield's excellent book "Multiple Exposures" chronicles these and describes the effects of radiation on health.
There are numerous books and web resources available on poisons aimed at readerships with varying technical backgrounds. Some useful books are:
Elements of Murder by John Emsley (Oxford, 2005)
Molecules of Murder by John Emsley (RSC Publishing 2008)
A is for Arsenic: the poisons of Agatha Christie by Kathryn Harkup (Bloomsbury Sigma 2015)
The Arsenic Century by James C. Wharton (OUP 2010)