Nicotine is highly addictive and fulfils both the physiological and psychological measures of addictiveness - it causes compulsive behaviour and it directly affects the brain's reward pathways.

It is a relatively simple molecule that easily passes into the bloodstream. From there nicotine crosses the important membranes that separate the blood from the brain and can take less than seven seconds to reach the brain from the lungs.

Once in the brain, nicotine stimulates certain nerve cells that bring about an apparent state of heightened awareness. Smokers say that nicotine improves their reaction time and ability to pay attention.

The chemical also stimulates the release - or inhibits the degradation - of an important neurotransmitter in the brain called dopamine which is part of the body's "reward system". These reward pathways are believed to reinforce behaviours that are good for survival - from eating to sex - making you feel happy and at peace.

The involvement of nicotine in dopamine levels is almost certainly the root cause of its extreme addictiveness. Other animals, from mice to gorillas, can get addicted to nicotine, indicating that it generates a fundamental stimulation of this vital reward system.

Nicotine may also increase levels of endorphins - natural painkillers - and glutamate, which is part of the memory system. Taken altogether, nicotine stimulates feelings of craving and well being that are remembered each time a cigarette is smoked or even seen by an addicted person.

One of the features of nicotine is that the body can get accustomed to it as the brain tries to compensate for the overproduction of dopamine. This means that greater amounts of nicotine are needed to produce the same levels of euphoria.

The chemical also stimulates other parts of the body, notably the release of the hormone adrenaline, which increases heart rate, blood pressure and breathing rate. Levels of glucose in the blood can also increase as a result of smoking, raising perceived energy levels.

Some scientists believe that nicotine is as addictive as heroin. However, nicotine withdrawal is relatively mild compared to the effects of stopping heroin abuse.

Less than 10 per cent of people in any one year who try to give up smoking succeed. Experts say that the best measure against nicotine addiction is preventing the young from smoking. Once addicted to nicotine, it is difficult but not impossible to stop.