Smoking may cause an increased risk of depression, according to a New Zealand study which has followed 1200 people since they were born in 1977.

Smoking and major depression have long been known to be linked.

But which is the primary cause has been unclear, and it is possible that they reinforce each other.

Studies have suggested that some people who have depression smoke as a form of self-medication.

Researchers from Otago University at Christchurch, led by Professor David Ferguson, analysed data collected when participants in their study were in the late teens to mid-20s.

They concluded it is probably smoking that increases the risk of suffering symptoms of major depression, rather than the reverse.

They found that at the age of:

* 18 years - 14 per cent of the study participants were addicted to nicotine and 18 per cent had serious depression.

* 21 years - 25 per cent were addicted and 18 per cent had depression.

* 25 years - 23 per cent were addicted and 14 per cent had depression.

"Overall, those reporting at least five symptoms of nicotine dependence had rates of depressive symptoms that were 2.13 times those of individuals who reported no symptoms of nicotine dependence," says the researchers' paper.

The findings were published yesterday in the British Journal of Psychiatry.

The paper says statistical calculations suggested the cause-and-effect link was one-way, from nicotine addiction to depression.

They found no evidence of a link in the reverse direction, from depression to smoking.

"The reasons for this relationship are not clear," said Professor Ferguson. "However, it's possible that nicotine causes changes to neurotransmitter activity in the brain, leading to an increased risk of depression."

Smoking is linked to an increased risk of:
* Cancer
* Heart disease
* Stroke
* Impotence
* Probably severe depression