But the researchers also found that people with high nicotine dependence were more likely to be readmitted to hospital with poor outcome to their condition, reported the Scottish daily Scotsman.

The study noted that worldwide, schizophrenia was linked to a higher rate of smoking than was seen in the general population or among people with other severe types of mental illness.

There have long been conflicting theories about whether smoking has any beneficial effect on symptoms of schizophrenia, which include delusions, hallucinations and depression.

The researchers looked at 250 patients in two community mental health centres - 69 per cent of whom were current smokers.

They found that some symptoms were less frequent in mildly dependent smokers compared to non-smokers or highly dependent smokers.

But for other symptoms - such as depression - they found no link with smoking at any level.

Those patients who were highly dependent on nicotine had more hospital admissions compared with those who did not smoke or who were only mildly dependent.

"If there is any beneficial effect of nicotine, it may be restricted to mildly dependent smokers, and particularly to those on low dosages of typical anti-psychotics," one of the researchers was quoted as saying.

"Such a benefit appears to affect only certain symptoms.

"Our study does not support the self-medication hypothesis for highly dependent smokers, who have poorer outcomes despite their heavy smoking."