Anti-inflammatory drugs similar to those used to treat rheumatoid arthritis and the skin condition psoriasis could be effective against some forms of depression, a study suggests.

Scientists analysed data from 20 clinical trials involving the use of anti-cytokine drugs to treat a range of auto-immune inflammatory diseases.

When they looked at the drugs' side effects, they spotted significant signs of them acting as antidepressants.

Cytokines are signalling proteins produced by immune cells that help the body fight off infection, but they can also cause harmful inflammation.

Previous work from the same team found that children with high everyday levels of one type of cytokine are at greater risk of developing depression and psychosis in adulthood.

This suggests that the immune system and low-grade body-wide inflammation can play a role in mental illness.

Study leader Dr Golam Khandaker, from Cambridge University, said: "It's becoming increasingly clear to us that inflammation plays a role in depression, at least for some individuals, and now our review suggests that it may be possible to treat these individuals using some anti-inflammatory drugs.

"These are not your everyday anti-inflammatory drugs such as ibuprofen, however, but a particular new class of drugs."

Anti-inflammatory drugs may offer hope for patients for whom current antidepressants are ineffective, the scientists believe.

Dr Khandaker added: "About a third of patients who are resistant to antidepressants show evidence of inflammation, so anti-inflammatory treatments could be relevant for a large number of people who suffer from depression."

The research is reported in the journal Molecular Psychiatry.