The World Health Organisation estimates that over a billion people world wide smoke cigarettes. It is well documented that there is a link between nicotine and several serious illnesses and potentially fatal health problems, with around 5 million people dying from a smoking related illness each year. But because nicotine is incredibly addictive most people will struggle to give cigarettes up, even if they are well aware the habit could kill them.

When a smoker inhales, the nicotine enters the body via the alveoli in the lungs. This means it quickly flows to the brain through the blood stream. This will only be a very small amount of nicotine as most is burnt away with the cigarette or just not absorbed into the bloodstream, but it is sufficiently potent to trigger alterations in the brain's chemistry.

At the core of the problem it causes by entering the brain, is the effect it has on he receptors. Usually the receptors seek out and bind to a neurotransmitter called acetylcholine, which serves several essential functions. But when nicotine enters the brain it binds to the receptor proteins instead and interferes with whatever function the target neurone was required for. For instance, if the neurone was involved with the nervous system it triggers a release of noradrenaline which then has a stimulating effect. The body reacts as it would during a 'flight or fight' response - heart rate increases, attention is improved and appetite is suppressed.

Nicotine also causes a surge of dopamine and this is what gives cigarettes their addictive quality. Most stimulants or drugs which are addictive cause an abnormally high level of dopamine in the pleasure pathway of the brain, which gives a feeling of well being and euphoria. But if used regularly the brain will start finding ways to diminish the sensitivity of neurones to dopamine to normalise the levels. This means that if a person suddenly stops using that artificial stimulant - in this case cigarettes - the dopamine levels will fall to unnaturally low levels because the brain is no longer so receptive to the neurotransmitter. Low dopamine levels can trigger mood disorders such as depression. Eventually, the body will readapt and dopamine levels return to normal, but only if the person is able to resist the cravings to restart their habit.

The best way to quit smoking is to do it gradually. Nicotine replacement therapy is very helpful as it gives the brain time to adapt. Good support from medical professionals or friends and family is essential to success when quitting smoking.