Grapefruit can increase the risk of side effects from some drugs
A woman who went on an intense grapefruit-based diet developed a blood clot in her leg and risked losing the limb, US doctors have reported.
The unusual case, written up in the Lancet medical journal, occurred in Washington state in November last year.
Medics concluded grapefruit had affected the way the 42-year-old's body processed her contraceptive pill.
A UK expert stressed this was an unusual case, but said extreme diets may have "unpredictable consequences".
In November 2008, the woman came to the casualty department of the Providence St Peter Hospital in Olympia, Washington state.
The day before, she had gone on a long car journey, after which she felt pain radiating from her lower back down to her left ankle.
When she arrived at the hospital she was experiencing difficulty walking, shortness of breath, and light-headedness.
By the next day her left leg had turned purple.
The woman was generally in good health but was slightly overweight and had decided to diet.
Three days before falling ill, she had begun a crash diet which included eating 225g of grapefruit each morning, after rarely eating the fruit in the past.
When doctors examined her, an ultrasound scan confirmed the woman had a large blood clot within the veins of her left leg, which stretched from her hip down to her calf and she was deemed to be at risk of losing her leg because of gangrene.
The woman was given clot-busting treatment and had a stent, a kind of tube, fitted in order to widen her vein.
The doctors treating her said a number of risk factors had contributed to the woman developing the clot.
She had an inherited disorder which increased her risk, as did being on the combined Pill. Being immobile in a car probably also contributed to the clot forming.
Writing in the Lancet, the authors led by Dr Lucinda Grande, called it a "constellation of potential risk factors".
But they added: "The increased [oestrogen] serum concentration due to her three days of grapefruit for breakfast may well have tipped the balance."
They suggest the fruit blocked the action of a key enzyme that normally breaks down the form of oestrogen in her contraceptive.
Dr Trevor Baglin, a consultant haematologist at Addenbrooke's NHS Trust in Cambridge, said: "From this case study it appears as if the grapefruit enhanced the thrombotic effect of the contraceptive pill in the presence of a genetic predisposition.
"However, it is worth pointing out that this is a single case study and a very unusual case at that.
"I would suggest that any extreme diets should be avoided because they can have unpredictable consequences."
A spokesman for the Florida Department of Citrus - an executive agency of Florida government which markets, researches and regulates the state's citrus industry, said: "The Lancet report looks to be inconsistent with published scientific studies which indicate grapefruit does not cause a clinically significant interaction with oral contraceptives.
"We are aware of no validated evidence that grapefruit affects oral contraceptives, and they are generally considered to be safe to consume with grapefruit."