Nicotine in high doses acts as an effective nerve poison and can have a number of potentially harmful side effects. It is extremely physically addicting, though estimates on the exact degree of addiction range wildly from very low levels to those rivaling that of heroine or cocaine. If taken in large doses — larger than almost anyone is likely to achieve through smoking — it may induce severe nausea or vomiting. In small doses, it may increase blood pressure, which can prove harmful, or in very rare cases, be fatal to those with dangerous heart conditions.
A number of recent studies have strongly linked nicotine itself to various cancers. This means that in addition to the cancer risks posed by tar through smoking, the chemical itself increases a smoker's chances of developing cancer. It also means that even those who use patches and gums are raising their likelihood of getting cancer. This link is thought to be caused by a property of the chemical that retards the body's ability to slough off damaged cells, giving cancerous cells more time to develop.
The lethal dosage of nicotine for a 150 pound (68 kg) male is 60 mg. This is less than both arsenic and strychnine. American cigarettes contain approximately 9 mg each (compared with 19 mg in a New Zealand cigarette, for example), but after burning, only about 1 mg enters the body over the course of smoking an entire cigarette. While this results in amounts well below the lethal dosage, over time, this poison can weaken the immune system and cause fatigue and other minor maladies.
Much more enters the body through chewing tobacco and many patches and gums than through smoking cigarettes; nicotine levels should be monitored when using these methods of disbursement. While gums and patches have maximum recommended doses, chewers of tobacco should be aware of how much of the chemical they are sending directly to their blood stream. An average pinch of chew held in the cheek for half an hour provides as much as smoking three or four cigarettes.
Nicotine is also a very potent insecticide, used as a natural alternative to chemical pest control substances. In most marketed forms, it contains 40% pure nicotine sulfate, mixed with water and sprayed on to crops. When used in warm weather, it provides optimal results, breaking down quickly to non-toxic levels and allowing for wide-spread use on food crops, even very close to harvest.