But she will, at least theoretically, be able to testify before the Senate and House about the attack on the American mission in Benghazi, Libya, which killed four Americans, including Ambassador J. Christopher Stevens. She was not able to appear at a hearing in December because of her illness. Republicans, who have sharply criticized the Obama administration’s handling of the attack and its aftermath, had demanded that she appear to explain the department’s role, though in recent days they have modulated their request.

Mrs. Clinton’s blood clot formed in a large vein along the side of her head, behind her right ear, between the brain and the skull. The vein, called the right transverse sinus, has a matching vessel on the left side. These veins drain blood from the brain; blockages can cause strokes or brain hemorrhages. But if only one transverse sinus is blocked, the vein on other side can usually handle the extra flow.

In one sense, Mrs. Clinton was lucky: a clot higher in this drainage system, in a vessel with no partner to take the overflow, would have been far more dangerous, according to Dr. Geoffrey T. Manley, the vice chairman of neurological surgery at the University of California, San Francisco. He is not involved in her care.

The fact that Mrs. Clinton had a blood clot in the past — in her leg, in 1998 — suggests that she may have a tendency to form clots, and may need blood-thinners long-term or even for the rest of her life, Dr. Manley said.

One major risk to people who take blood thinners is that the drugs increase bleeding, so blows to the head from falls or other accidents — like the fall that caused Mrs. Clinton’s concussion — become more dangerous, and more likely to cause a brain hemorrhage. Even so, the medication should not interfere with Mrs. Clinton’s career, Dr. Manley said.