Heart disease patients in the UK taking the blood thinning drug warfarin have been warned to avoid cranberry juice, after reports that the drink increases the drug's potency.
The UK's Committee on Safety of Medicines says it has received five reports since 1999 showing that the juice had a significant effect on the drug.
One man died from a gastrointestinal haemorrhage after his blood clotting levels changed dramatically six weeks after starting to drink the juice.
Cranberry juice is often used by women to prevent cystitis. It has also been found in recent research to have benefits on heart health, with a recent trial showing that a cranberry extract reduced stroke outcome, and is therefore increasingly prescribed for patients after a heart attack or heart surgery.
However, the flavonoids in the fruit are known to inhibit cytochrome P450 activity, the enzymes used to break down warfarin. They have previously been found to interact with many other drugs too.
"Until this possible interaction between cranberry juice and warfarin has been investigated further, it would be prudent for patients taking warfarin to be advised to limit or avoid drinking it," the committee said.
This week has seen reports from both sides of the Atlantic warning of the potentially dangerous interactions between natural remedies and prescription medication.
Research presented at the British Pharmaceutical Conference in Harrogate revealed that a lack of consumer and health professional knowledge of herbals was putting people at risk. In the US, a study published in JAMA found that the herb St Johns wort, taken to treat depression and anxiety, could interfere with up to 50 per cent of medications taken by Americans, owing to its effects on CYP3A4 enzymes.