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Dear Dr. Roach My brother-in-law has had blood clots in his legs twice before. Now he has another one in his calf, and tests showed several in his lungs. What causes blood clots, and how can you prevent them? — M.K.E.

Answer • The body must have a robust ability to stop bleeding, which needs to be ready at any time. That ability has a cost: possible inappropriate clotting inside the body.

Most people will never have a problem with inappropriate clotting, but there are dozens of medical conditions that can increase the risk. Some of these run in families. Prolonged immobility, such as during a hospitalization or even on long plane rides, is a clear risk factor for blood clots. All types of cancer increase the risk of blood clots, as does surgery. A pregnant woman, or one taking estrogen, is at higher risk. People with some genetic conditions are at higher risk; one that’s common is the factor V Leiden mutation.

While there is nothing you can do about a genetic predisposition, there is some advice that can help people who are at risk for other reasons to avoid blood clots. For those people at lower risk, here are some practical tips for avoiding them:

• Stay active and avoid sitting for long periods of time. Get up once an hour, even if only for a minute.

• If you can’t move — for example, if the “seat belt” sign never gets turned off on a long plane ride — at least move your feet and work the muscles of your legs.

• Drink adequate water during travel.

People who have already had a deep vein thrombosis, a blood clot deep in the leg veins, should get specific medical advice about preventing another one, including possibly using medication.

Dear Dr. Roach • I am 91 and fairly healthy. The information on the safety of taking a daily 81 mg aspirin seems contradictory. What is the current thinking? — C.H.

Answer • Low-dose aspirin is recommended for people with known coronary artery disease, that is, blockages in the arteries of the heart. This is true whether or not they have had a stent or surgical bypass graft.

For those without known blockages, recent studies have shown that fewer people benefit from aspirin than previously thought, largely because rates of coronary disease and death have dramatically dropped in the decades since the initial trials on aspirin were published.

Some of the studies did not examine the benefit of aspirin on preventing colon cancer. The benefits of reducing both heart disease and colon cancer need to be weighed against the risks of bleeding. Since all of these risks vary among different people, and because an individual may have very strong feelings about particular risks, the decision of starting low-dose aspirin should be made after an individualized evaluation. People with a history of abnormal or serious bleeding are more likely to experience harm than benefit from aspirin therapy.

Bleeding risks tend to be higher in older people, even higher than the risk of heart blockages, so no expert groups recommend aspirin to prevent disease in healthy people over 70. It would be a very unusual situation to recommend aspirin to a person in his or her 90s without known coronary blockages.

Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.