What makes cigarettes addictive is nicotine, and the nicotine in tobacco is the root cause of addiction that is difficult for smokers to quit. Nicotine, tar, and CO in cigarette smoke are the three most harmful chemicals to health. Nicotine will make smokers dependent, nicotine can activate acetylcholine nicotinic receptor, promote the release of dopamine, resulting in addiction. Tar contains a variety of carcinogens, can inhibit immune function and induce cell mutation. Carbon monoxide decreases blood oxygen levels and increases blood viscosity. Withdrawal symptoms such as increased blood pressure, insomnia, arrhythmia, and anxiety usually occur when forced to quit smoking. E-cigarettes also contain nicotine, which is also addictive and does not help in quitting smoking, and it is recommended to stay away from e-cigarettes.