Secretary of State Hillary Rodham Clinton has been admitted to a New York hospital after the discovery of a blood clot stemming from the concussion she sustained earlier this month.
Clinton spokesman Philippe Reines said her doctors discovered the clot during a follow-up exam on Sunday.
Mr Reines said Clinton is being treated with anti-coagulants - but he would not indicate where the blood clot is located.
Health scare: Hillary Clinton's doctors discovered the clot during a follow-up exam on Sunday
Clinton was admitted to New York-Presbyterian Hospital so doctors can monitor the medication over the next 48 hours.
Dr. Larry Goldstein, a neurologist who is director of Duke University's stroke center, said blood can pool on the surface of the brain or in other areas of the brain after a concussion, but those would not be treated with blood thinners, as Clinton's aide described.
Aides and doctors say Clinton contracted a stomach virus in early December and became dehydrated, then fainted, fell and hit her head on December 9.
She was diagnosed with a concussion on December 13 and hasn't been seen in public since.
Political force: Clinton is known as President Obama's most popular cabinet members - and among the names mentioned for a possible 2016 presidential run
Medical care: Clinton was admitted to New York-Presbyterian Hospital so doctors can monitor her medication over the next 48 hours
The seriousness of a blood clot 'depends on where it is,' said Dr Gholam Motamedi, a neurologist at Georgetown University Medical Center who was not involved in Clinton's care.
Clots in the legs are a common risk after someone has been bedridden, as Clinton may have been for a time after her concussion.
THE CLUES ABOUT CLOTS
The new health scare may bring back some painful memories for the former first lady, who suffered a large blood clot in her leg back in 1998.
In a 2007 interview with the New York Daily News, Clinton called the 1998 clot 'the most significant health scare I've ever had.'
Those are 'no big deal' and are treated with six months of blood thinners to allow them to dissolve on their own and to prevent further clots from forming, he said.
A clot in a lung or the brain is more serious. Lung clots, called pulmonary embolisms, can be deadly, and a clot in the brain can cause a stroke, Motamedi said.
Clinton's illness led her to cancel an overseas trip and scheduled testimony before Congress about the attack on the U.S. Consulate in Benghazi, Libya.
When her absence was reported, several pundits and newspapers accused Clinton of making her illness seem worse than it was to dodge questions from lawmakers over the consulate attack, which claimed the lives of four Americans, including Ambassador Chris Stevens.
The New York Post called her concussion a 'head fake.'
Florida Rep Allan West said Clinton had a case of the 'Benghazi flu,' while Fox News contributer Charles Krauthammer dubbed it an 'acute Benghazi allergy.'
Reines said doctors will continue to assess Clinton's condition, 'including other issues associated with her concussion.'
Earlier this week, The National Enquirer reported that brain cancer was behind Mrs Clinton's health problems and that she was facing a barrage of medical tests to confirm the diagnosis.
But a spokesman for the 65-year-old Democrat labelled the claims 'absolute nonsense' and insisted Clinton was recovering well from the fall and subsequent concussion.
Only days before her concussion Clinton had said she was in excellent health during an interview with Barbara Walters.
At 65, detractors have claimed Clinton's advancing age and health make her too old to realistically serve as a two-term president were she elected in 2016.
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