A popular alternative eczema treatment called "starflower oil" has little impact on the condition, say doctors.
Severe eczema cases can be disabling
Patients given the extract fared no better than those trying a placebo drug, experiments found.
The study, published in the British Medical Journal, is also bad news for those using Evening Primrose Oil to tackle eczema.
It shares the same ingredient as starflower oil, though in different concentrations.
"Starflower oil" is actually an extract of borage, a herb which grows in the UK.
It has been suggested that it could have an anti-inflammatory effect which could ease the symptoms of eczema.
Doctors are looking for alternatives to potentially harmful steroid treatments for eczema.
The trial was run from George Eliot Hospital in Nuneaton, and involved 151 patients, half given starflower oil and half the placebo.
After 12 weeks, their eczema symptoms - skin rashes - were reviewed to check for signs of improvement.
There was a slight improvement in the starflower oil group - but a bigger improvement was noticed among the placebo group, who had received no herbs or medicines at all.
There was no evidence that those on starflower oil used a smaller quantity of steroids as a result.
The conclusion was a plain one - that linolenic acid, the principal active ingredient, was "not beneficial".
Professor Hywel Williams, from the Centre of Evidence-based Dermatology at Queen's Medical Centre in Nottingham, said that the largest and best studies on the subject of gamma linolenic acid, the ingredient of starflower and evening primrose oils, had shown no convincing evidence of benefits against dermatitis.
"Nobody would have been happier than myself if evening primrose oil had produced a clinically worthwhile benefit for eczema sufferers.
"But the history of its development has been marred by lack of data in the public domain."
He added: "As we bid goodnight to the evening primrose oil story, perhaps we can awaken to a world where all clinical trial data, derived from people who are good enough to volunteer for these studies, reach the light of day, where they can be openly debated."