"Everything is poison, there is poison in everything. Only the dose makes a thing not a poison."
A poison is a substance that causes harmful metabolic disturbance to a given individual - although the term is often extended to substances that cause tissue damage (technically corrosives) and genetic disturbances (many technical terms including mutagen, carcinogen and teratogen, depending on their exact mode of harm).
As Paracelsus notes in the flavour text, pretty much anything can act as a poison in the right dose - and a lot depends on the species (and, indeed, the individual genetics) of the target. Also, since harm is in the eye of the beholder drugs of all kinds can normally be considered poisons as well.
This level of diversity makes describing and classifying poisons something of a mission and for most purposes the definition can be restricted to substances causing permanent damage at a relatively low dose. These can be classified as follows:
Biological vs. Chemical:
Technically all poisons are chemicals, but this refers to the method of production - is the substance isolated from a living creature or manufactured from inanimate chemicals? Whilst synthetic chemistry has a style all of its own, living creatures can come up with an impressive variety of poisons from venoms and antifeedants to the sort of toxic wasteproducts created by bacteria like c.botulinum.
By method of delivery:
This is probably one of the main bases on which a poison would be purchased - the usual routes of delivery are contact (across unbroken skin), injection (through a wound), ingestion (via the digestive tract) and inhalation (via the lungs). All have their requirements - an ingested poison must have a mild enough flavour that the target fails to notice it until they've swallowed a lethal dose, an inhaled poison is probably dangerous to handle, store and use and contact poisons have to possess a rare triple combination of toxicity, stability and the ability to permeate skin. What is required of an injected poison depends further on the method of injection - if it's delivered in an airgun dart, it will be less demanding to formulate than a poison that needs to stick to the cutting edge of a sword in order to be delivered. Other methods than "the big four" exist, but are marginal and in some cases an excipient like Dimethyl Sulphoxide can transform a poison from one category to another1.
Does the poison kill or incapacitate its target? What specific organs does it target? How quickly does it work and how big a dose do you need?
You may also be interested in ideas like the existence of an antidote to the poison - in case you dose yourself by mistake - or more complex devices like a binary poison, which is only harmful (or at least much more harmful) if two seperate substances are consumed. This allows a poisoner to share a meal with his host, including sharing one of the parts of the poison, so long as he avoids the other. This is mainly a feature of ingested poisons, although modern chemical weapons also use binary agents to generate some of the more aggressive nerve gases.
Another interesting feature of poisons is that some (but not all) of them can be tolerated by taking small but steadily increasing doses in order to build up an accquired immunity. This was the much quoted speciality of Mithridates of Pontus … and ironically significantly impaired his ability to kill himself when he fell into Roman hands at the end of the Mithridatic War. By contrast attempting this with the wrong sort of poison - especially those that accumulate in the human body - can greatly expedite suicide.
Of course not all poisoning is deliberate - many people are killed as a result of the accidental or mistaken consumption of poisonous substances or those that have been naturally contaminated by decay organisms and many more by the careless or ignorant misuse of dangerous substances.
Game and Story Use
- Poison will appear a lot in most RPGs - some of the sillier ones will call the use of poison "evil", but that is a rules mechanic best given the respect that it deserves. What may be appropriate is to consider that a lot of "honour" based cultures regard the use of poison as cheating and react very badly to it - this means that some character types will probably refuse to use it, even when it would be ethically superior to do so.
- The species variability on some poisons should be carefully noted - even to the extent that some creatures that naturally feed on poisonous plants or animals may themselves be toxic, despite not being harmed by what they are eating. Actually, even parts of some otherwise edible animals are toxic for one reason or another (e.g. polar bear liver, toxic due to high concentrations of fat soluble vitamins).
- There are things … undead, demons and devils, constructs and the like - that probably can't be affected by normal poisons. That doesn't necessarily mean that they can't be poisoned at all, just that the PCs will need to dig a little deeper to find something to use.
- Traps are a traditional source of poisons.
- Many RPG mechanics handle poisons very badly and make them either unrealistically deadly ("save vs. poison or die") or unrealistically weak (involving minor status effects).
- Buying poisons is likely to require the right connections - a physician or alchemist can probably buy the requisite substances for many common poisons without too much question … although those poisons will probably be easier to diagnose (and maybe treat) and most apocatheries are liable to remember anyone who bought a significant quantity of a known poison. Amateur poisoners are liable to encounter more obstacles. Rarer poisons may be very hard to find indeed.
- An assassain's guild … if such things exist … is liable to be the best source of poison experts going.