Nicotine is the main substance leading to tobacco dependence. Nicotine is fat-soluble, and can quickly pass through the blood, blood-brain barrier and central nervous system after being inhaled into the lungs, resulting in biological effects.

The secret of nicotine addiction mainly lies in reward effect and dopamine. Dopamine reward circuit located in the limbic system of midbrain is most closely related to drug dependence. Nicotine can activate nicotinic acetylcholine receptors in the brain, promote the release of dopamine, and make smokers feel "happy". The half-life of nicotine is 2~3 hours. If the addict stops smoking, the nicotine concentration in the body will drop rapidly, and he can't continue to experience the feeling of "pleasure" and appear withdrawal symptoms. In fact, the pleasure of nicotine after addiction is very limited, and smokers actually continue to smoke just to avoid the discomfort of withdrawal symptoms. In order to avoid withdrawal symptoms, smokers need to smoke at short intervals.

Nicotine itself is not the most harmful substance in tobacco, but because it leads to addiction, smokers will continue to smoke, thus causing health damage to themselves and people around them.

Nicotine also has some effects on health, for example, it can excite sympathetic nerve, which leads to an increase in heart rate, which is related to vascular endothelial dysfunction, lipid abnormality and insulin resistance (related to cardiovascular and metabolic diseases).