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.
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".
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.
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."
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."