A daily dose of specially-formulated dark chocolate may help cut chronic fatigue syndrome symptoms.
The reduction was small but "noteworthy", researchers say
Patients in a pilot study found they had less fatigue when eating dark chocolate with a high cocoa content than with white chocolate dyed brown.
Researchers from Hull York Medical School said the results were surprising but dark chocolate may be having an effect on the brain chemical serotonin.
Experts said patients should consume chocolate in moderation.
Chronic fatigue syndrome (CFS), also known as ME, is a condition with a diverse range of symptoms but particularly characterised by profound muscle fatigue after physical exertion.
Study leader Professor Steve Atkin, an expert in endocrinology, said the idea for the study came after a patient reported feeling much better after swapping her normal milk chocolate for dark chocolate with a high cocoa solid content.
He decided to see if other patients would benefit and carried out a trial of 10 patients who received a daily dose - 45g - of dark chocolate or white chocolate dyed to look like dark chocolate for two months.
The patients then had a month off before taking the other type of chocolate for two months.
Those taking dark chocolate reported significantly less fatigue and reported feeling more fatigue when they stopped eating it.
Professor Atkin said he was very surprised at the strength of the results.
"Although it was a small study, two patients went back to work after being off for six months."
He explained: "Dark chocolate is high in polyphenols, which have been associated with health benefits such as a reduction in blood pressure.
"Also high polyphenols appear to improve levels of serotonin in the brain, which has been linked with chronic fatigue syndrome and that may be a mechanism."
He added that although more research was needed to confirm the findings, patients would not do themselves any harm by eating small amounts of dark chocolate and no-one in the study put on any weight.
"If you derive benefit, then it's a no-harm, no-risk situation."
Jane Colby, executive director of The Young ME Sufferers Trust said it was important to distinguish between ME and other types of fatigue.
"But a little bit of what you fancy does you good - if it's not doing you any harm and it seems to be helping you then fair enough but I don't think it's an instant cure."
She added that people needed to eat chocolate in moderation to ensure they do not put on weight.
Heather Walker, Communications Manager, Action for ME, said: "Wouldn't it be wonderful if eating chocolate every day could alleviate the symptoms of chronic illness?
"If it were that easy, there would not be 250,000 people in the UK today whose lives are being been devastated by ME."
The researchers stressed the chocolate formulation used in the study was not currently available to the public.