This article by Bernd Mayer (Department of Pharmacology and Toxicology, Karl-Franzens University Graz) states, "Nicotine is a toxic compound that should be handled with care, but the frequent warnings of potential fatalities caused by ingestion of small amounts of tobacco products or diluted nicotine-containing solutions are unjustified and need to be revised in light of overwhelming data indicating that more than 0.5 g of oral nicotine is required to kill an adult."

Read the full article here:

http://link.springer.com/article/10.1007%2Fs00204-013-1127-0/fulltext.html