Why is smoking addictive?

Cigarettes contain nicotine, which is highly addictive.
Even if you want to quit smoking, you may find it difficult because you’re addicted to the effects of nicotine. Some research has suggested that nicotine can be more addictive than heroin.

Chemicals in your brain

Nicotine alters the balance of chemicals in your brain. It mainly affects chemicals called dopamine and noradrenaline. When nicotine changes the levels of these chemicals, your mood and concentration levels change. Many smokers find this enjoyable.
The changes happen very quickly. When you inhale the nicotine, it immediately rushes to your brain where it takes effect to produce feeling of pleasure and reduces stress and anxiety. This is why many smokers enjoy the nicotine rush and become dependent on it.
The more you smoke, the more your brain becomes used to the nicotine. This means that you have to smoke more to get the same effect.

Effects of quitting smoking

When you stop smoking, the loss of nicotine changes the levels of dopamine and noradrenaline. This can make you feel anxious, depressed and irritable.
It’s normal to crave nicotine when you quit, as smoking provides an immediate fix to these problems.

Getting help to quit

Nicotine cravings can be very strong, making it difficult to quit using just your willpower.
If you want to stop smoking, see your GP who can refer you to an NHS Stop Smoking support service.
These services offer the best support for people who want to give up smoking. Studies show that you’re four times more likely to quit smoking if you do it through the NHS.
These programmes can provide nicotine substitutes, such as patches and gum or medicine treatments, to help you stop smoking. They also provide counselling, support and advice.

refference : http://www.nhs.uk/chq/Pages/2278.aspx?CategoryID=53