The title of this node is almost correct. There is sometimes a thin line
between what is a poison/drug and what is not. I think most people, however would be happy and debate free if we take arsenic
as a poison and not a drug (although it has drug like properties), and we can call THC
a drug and not a poison.
Now if we draw the line between calling something a drug/food and calling it a poison based on the compounds toxicity, then we can start to make some judgements. We'll measure our toxicity using good old LD50 in rats. That is to say, in a given test, how much of a substance will kill 50% of test subjects. We'll draw the line and say that DDT is a poison and that aspirin is a drug. Why drawing the line there? For the purposes of debate.
Sand Jack pointed out that sugar affects the nervous system. Well the LD50 for rats in the test we are using shows us the toxicity of sucrose is 29,700mg/kg. Sugar certainly isn't a practical poison.
- Aspirin: 1000mg/kg
- DDT: 113mg/kg
- Arsenic (arsenic acid) 48mg/kg
- nicotine: 1mg/kg
If we consider a poison to be a measure of relative toxicity and that DDT and arsenic are poisons, then nicotine is certainly a poison. And of course, most toxicologists and lexicographers would agree.
Unfortunately, there is no line between poison and drug. Many substances, which have a high toxicity are used at low dosages for recreational purposes. LD50 for rats using LSD is about 16.5mg/kg. Something can be both a poison and a drug. The definition of a drug is a little hazy and poison is a little more defined. Either way, nicotine is highly toxic, and a nicotine related death is as unpleasant as it is rare.
To conclude, you may argue that nicotine isn't a drug, by what you define what a drug is. To not consider it a poison, however is a little harder to justify. Nicotine is addictive, but then so is Arsenic, and Arsenic has more of an "effect" (it gets you higher).
Note, the figures used above, vary by a factor as much as 10 from test to test. My figures came from "Toxicology for the Citizen" produced at Michigan State University