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That new symptom is troubling: the inexplicable swelling in your calf or the blood in your urine. Could it be serious or even life-threatening?

"Your body flashes signals -- symptoms and signs -- that warn you of potential problems," say Neil Shulman, MD, Jack Birge, MD, and Joon Ahn, MD. The three Georgia-based doctors are the authors of the book Your Body's Red Light Warning Signals.

Fortunately, many symptoms turn out not to be serious. For example, the majority of headaches stem from stress, eyestrain, lack of sleep, dehydration, caffeine withdrawal, and other mundane causes.

But a sudden, agonizing "thunderclap" headache -- the worst of your life -- could mean bleeding in the brain. Being able to recognize this serious symptom and calling 911 may save your life.

Here are six important flashing signals.

1. Paralysis of the arms or legs, tingling, numbness, confusion, dizziness, double vision, slurred speech, trouble finding words, or weakness, especially on one side of the face or body.

These are signs of stroke -- or a "brain attack" -- in which arteries that supply oxygen to the brain become blocked or rupture, causing brain tissue to die.

Symptoms depend on which area of the brain is involved. If a large blood vessel is blocked, a wide area may be affected, so a person may have paralysis on one side of the body and lose other functions, such as speech and understanding. If a smaller vessel is blocked, paralysis may remain limited to an arm or leg, or even the face.

If you have symptoms, call 911 right away and get to an emergency room that offers clot-busting therapy for strokes due to blocked vessels. Such treatment, which dissolves clots in blocked vessels, needs to be given within the first 3 hours after symptoms begin, but newer treatments may work within a longer time frame, says Birge, who is medical director at the Tanner Medical Center in Carrollton, Ga.

Timing is urgent; fast treatment can potentially stop brain tissue death before permanent brain injury happens. "There is a time clock ticking as to when you might totally recover," Birge tells WebMD.

2. Chest pain or discomfort; pain in the arm, jaw, or neck; breaking out in a cold sweat; extreme weakness; nausea; vomiting; feeling faint; or being short of breath.

These are signs of heart attack. If you get some of these symptoms, call 911 immediately and go to the emergency room by ambulance. Shulman and Birge also recommend that patients chew one regular, full-strength aspirin (unless they're allergic to aspirin) to help prevent damage to the heart muscle during a heart attack.

Not everyone who has a heart attack feels chest pain or pressure or a sense of indigestion. Some people, especially women, the elderly, and people with diabetes, get "painless" heart attacks, the doctors say. Being aware of "painless" heart attack signs is crucial: symptoms may include weakness, sudden dizziness, a pounding heart, shortness of breath, heavy sweating, a feeling of impending doom, nausea, and vomiting.

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