Previous studies on the anti-depressant Zyban have shown that it doubles the chance of smokers being able to give up.

Now new research shows other anti-depressants may have a similar effect.

Sharon Hall, professor of psychiatry at the University of California in San Francisco has conducted a study using the anti-depressants Aventyl and Pamelor (the brand names for nortriptyline).

According to Science News, in the study of 199 people who wanted to stop smoking, some were given placebos and others anti-depressants.

All were given education sessions on the benefits of giving up smoking.

After a year, urine tests showed that 25% of the people on placebo drugs were smoke free.

But 40% of those on the anti-depressants were still off cigarettes.

Many of these also stated that they experienced a feeling of well-being on giving up smoking rather than the usual withdrawal symptoms.

Addiction

A University of Florida study, published earlier this year, showed that anti-depressants could help treat the root cause of addiction.

Anti-depressants may be effective in treating the cause of addiction

It said long-term smoking changed the chemical make-up of the brain, leading the body to expect constant nicotine highs.

The university attempted a three-pronged attack on cigarette addicts, using counselling, nicotine patches to treat acute withdrawal symptoms and anti-depressants to tackle long-term addiction.

Another reason why anti-depressants are likely to help people stop smoking is that research shows a strong link between depression and smoking.

Accident

The first anti-depressants - monoamine oxidase inhibitors - were discovered in the 1960s - by accident.

Researchers were looking for a drug to treat tuberculosis. Instead they hit on a formula for a drug which lifted people's mood.

The first anti-depressants had many side effects. Newer drugs tend to have fewer effects and many of the effects, such as a dry mouth and headaches, disappear after the first weeks of use.

Other effects include loss of libido.

The drugs work by protecting the brain's chemical messengers or neurotransmitters.

Depression is linked to low levels of neurotransmitters, especially serotonin, epinephrine and norepinephrine.

Different anti-depressants treat different symptoms and some people respond better to one type than another.

Usually the effect is felt within two to four weeks.

Controversy

Proponents of the drugs say they work for between 60% and 80% of people diagnosed with depression.

But a recent study published in the New Scientist said the effectiveness of using anti-depressants could be due almost entirely to patients' expectations that they are going to work and their relief at being treated.

The US study said anti-depressants and sedatives such as Prozac could be at best only 25% more effective than placebos.

Previous research had suggested they could be up to 40% more effective.