About 300 people with a persistent cough are taking part in a clinical trial at 13 National Health Service hospitals in the U.K., where they are being given the naturally occurring chemical theobromine, derived from the raw ingredient of chocolate, twice a day for 14 days. Early indicators are that 60% of patients experience some measure of relief.

Researchers said a daily bar of dark chocolate may contain enough of the active compound to have an effect on a chronic cough, but it is not a cure since symptoms did return once treatment was ended.

The amount of the theobromine used in the trial was a single dose of 1,000mg. Unsweetened dark chocolate has about 450mg per ounce, sweet dark chocolate around 150mg and milk chocolate about 60mg.

"This new capsule we are using seems very effective," principal investigator professor Alyn Morice, head of the Hull Cough Clinic, said.

"Eating a bar of dark chocolate a day which has high levels of the compound may also be effective for people with diagnosed persistent cough, although eating chocolate on a daily basis may have other unwanted effects, including weight gain and so on," Morice said.

An earlier study at the National Heart and Lung Institute showed that theobromine appears to block the action of the sensory nerves, which in turn halts the cough reflex. It was found to be more effective than widely used codeine.