Gosh I read through this entire thread before I decided to post. I have tons to say and I am not sure I could even begin to start to organize my thoughts. I tend to ramble on so I will try to be as succinct as possible. OK, not really possible.
To begin with the OP asked: how toxic nicotine is and how addictive it is then another poster added the question How much does nicotine help treat millions of people?
It is toxic but so is iron and most substances. Give anyone too much iron and you will kill them. Our bodies are designed with a nicotinic receptor in our brains. So at some level a controlled amount of nicotine is biologically acceptable to our bodies. People need to remember that our ancestor's have been smoking or otherwise consuming tobacco and other nicotine containing herbs for hundreds of thousands of years. The chemical make up of nicotine and niacin (b3) are structurally so similar that scientists call niacin, nicotinic acid. It may just be that we have evolve to safely consume nicotine at a reasonable level.
As far as asking how addictive is nicotine, I believe it is a fallacy that we can quantify that. The term addiction in itself is a problematic word in that most people do not understand exactly what it means. You can be clinically dependent (physical dependence) but not addicted in our ordinary language. They use that term because we can measure physical dependence. But there is no way to measure psychological dependence (aka addiction) because that varies due to peoples personalities, culture, social acceptability etc. For example it may be completely acceptable in some societies to ingest mind altering things as a religious experience. In that culture you would never consider that person addicted. The definition of addiction therefore is a judgment in the eye of the examiner. So to make my answer short, can nicotine be addictive, no. Can it be something you can become dependent on yes.
Lastly, in 1994/95, I did a research paper in college on irritable bowel syndrome and the above nicotinic receptor. Researchers found that IBS was successfully treated with nicotinic acid (niacin) and nicotine in a few small studies. This of course was before smoking was a huge public problem. I don't think any researcher these days would be brave enough to suggest or even test that nicotine would be helpful. Sadly, they are taking out of the picture a chemical that our bodies naturally have a receptor for.
