One of the reasons quitting is so difficult is the highly addictive nicotine found in cigarettes. Over time, a smoker becomes both physically and emotionally dependent on their habit. When they inhale, the nicotine in the smoke quickly enters the bloodstream, reaching the brain in a matter of seconds. Here, it interacts with specific receptors to trigger feelings of pleasure. Over time, the body adapts to the level of nicotine and the smoker develops a tolerance for these effects. They find themselves having to smoke more to get the feelings they desire.

When they try to cut back or quit, the body 'calls out' for nicotine, leading to withdrawal symptoms. Withdrawal symptoms can start within a few hours of the last cigarette and tend to peak two to three days later. Symptoms may persist for anything from a few days to several weeks and can include headache, irritability, restlessness, difficulty concentrating and insomnia. Quitters may also experience depression, anxiety, increased appetite and dizziness. Psychological symptoms may also occur as smokers associate certain situations with having a cigarette, resulting in temptation. These so-called situational cravings can be very intense and greatly challenge a person's willpower.