Bukujutsu, to try to answer your question: Nicotine has very high bioavailability when absorbed through ANY skin, so I would imagine that under the tongue would have a really great bioavailability, similar to vaping or smoking it. Because it is absorbed so well, BE VERY CAREFUL. You can easily overdose on pure nicotine just by spilling it on yourself or something. Make sure that you are measuring the amounts very carefully. Do you have a good knowledge of titration? If not, then maybe you should buy e-cig juice and use that to be a bit safer.
Also, nicotine isn't entirely harmful. The carcinogenic effects of it are VERY low compared to consumption of any other kind of "full" tobacco. With things like dip or cigarettes, the main carcinogens come from additives or combustion products. It does have some cancer-promoting effects, but there appears to be a very small risk. There even appear to be very useful applications of nicotine in mental illness.
The largest risk you run by subscribing to any theory of nicotine is the risk created by study manipulation. Tobacco companies are very powerful and have a large influence in scientific studies. They also own the methods of quitting (excluding e-cigs). There is also a very strong group of people opposing cigarette use or hoping to gain tax revenue by demonizing cigarettes. Because of these biases, the evidence is very muddled and always has been on the topic of tobacco. It is hard to tell which studies on tobacco are legitimate.
That being said, I tend to lean toward the theory that nicotine itself is not very harmful (and even that the harm of tobacco is largely over-hyped, but still real enough that you shouldn't take up smoking).
Edited by brainslug, 19 May 2013 - 02:38 AM.