Plants that belong to nightshade (like tobacco, tomato, potato, aubergine and green pepper) produce special insecticide as a way of protection from insects eating them. It is called nicotine. But because of the nicotine action on human brain people started to use it intensively even though it is a very poisonous substance (it is three times more lethal than arsenic).
Nicotine is an oily substance that dissolves in water. The highest amount of nicotine is found in tobacco. The actual synthesis of nicotine happens in roots of the tobacco plant but nicotine reaches its highest concentration in plant's leaves (it can be up to 3-5 % in dry leaves). That is why most of the nicotine containing products is made of tobacco leaves. Nicotine is absorbed easily so these products do not have to be smoked as there are many smokeless tobacco products. The name "nicotine" came from Latin name of tobacco plant - "Nicotiana tabacum". And the substance was firstly isolated in 1828. That was made by Germans - Posselt and Reimann.
Nicotine action can be different depending on dosage of the substance and the amount of nicotine a person has previously received. In small concentration nicotine is acting as a stimulant - increases activity and memory. Nicotine also increases the heart rate and blood pressure.
Because of the nicotine action on human brain repeated use causes strong physiological dependency. As long as the nicotine action is pretty similar to effect of neurotransmitter acetylcholine it increases dopamine level in the brain. Recent studies also show that nicotine inhibits monoamine oxidase. And all these processes give the sensation of pleasure similar to that caused by the use of heroin or cocaine. So, people continue to smoke tobacco to keep their dopamine levels high. And lack of the nicotine is acting as a sedative for these people as they become depressed.
Other nicotine withdrawn symptoms also include irritability and anxiety. In fact, nicotine action is pretty short and it lasts for about half an hour so smokers need to refill themselves with nicotine very often. And what is even worse here is that body gets tolerant to nicotine action. So, there is a constant need to increase the dosage of nicotine needed to get the same sensation.
But as long as nicotine is a natural poison it can seriously harm general health. The main danger of continuous use of nicotine is that it lowers the activity of apoptosis which is the main mechanism that body uses to destroy mutated cells. This means that nicotine use can increase the possibility of cancer. In that sense nicotine should be considered to be a strong carcinogen.
The best method to break nicotine dependency is to act slowly. There are different products that allow keeping nicotine at the controlled level and at the same time lowering the daily dosage. Different dermal patches, gums or nasal sprays are offered by pharmaceutical industry. Special studies in the area of nicotine action show that it can be used in treatment of autosomal dominant frontal lobe epilepsy.