DEAR DR. DONOHUE: My husband has a blood clot in a leg vein. It starts at knee level and goes upward. It is in a large vein in the inside. He is on a blood thinner for six months.

Lately we have heard of a few others with the same problem, and their doctors say to stay on blood thinner for the rest of their lives. Should my husband do the same?

Is it possible to do a vein bypass in the leg, like they do for the heart? Is this surgery successful? – Anon.

ANSWER: By “inside,” you mean a “deep” leg vein, one that is buried in the leg muscles, not one under the skin.

Clots in deep leg veins can cause great trouble in addition to the pain they generate. Pieces of the clot can break free from the main body of the clot. They’re like icebergs traveling in the circulation and landing in the lungs. If a clot piece lodges in a lung blood vessel, it cuts off blood supply to that section of the lung, and that lung section dies. The floating clot is called an embolus, and when it strikes the lung it is a pulmonary embolus. A large pulmonary embolus can be deadly.

For that reason, doctors put people with such clots on blood thinners. The blood thinner prevents the original clot from enlarging and lessens the chances that emboli will break free from it.

Ordinarily a person with a clot in one of the deep leg veins continues on the blood thinner Coumadin for six months. By that time a new channel should have formed in the vein. People who have had a second clot in a deep leg vein often must take Coumadin for life, as do people who have an innate clot-forming tendency. Such people have a defect in their normal clot-dissolving mechanism.

They are a minority of clot patients.

Leg veins are not usually surgically bypassed as heart arteries are.

DEAR DR. DONOHUE: I am 54 and have been through menopause. I haven’t had a period for more than two years. When I did have them, I suffered terribly because I had endometriosis. I have two daughters, 23 and 18. Are they likely to come down with endometriosis because I had it? – M.M.

ANSWER: The endometrium is the uterine lining. In endometriosis, bits of the uterine lining have migrated to places outside the uterus – in the pelvis and on the ovaries, for example. Those bits of tissue respond to the monthly surge of female hormones just as the endometrium in the uterus does. This can cause women great pain, particularly during the menstrual cycle. Transplanted endometrium has no natural exit from the body, as uterine endometrium has. Not only can endometriosis cause pelvic pain, but it can make intercourse most uncomfortable, and it can lead to infertility.

It does have a genetic basis. If the mother had or has endometriosis, her daughters have a two to 10 times greater risk of getting it than do the daughters of women without the condition. It usually is diagnosed between the ages of 25 and 35, so your daughters have a few years to go before they will know if they have inherited the propensity for it.

The endometriosis booklet discusses this topic in great detail. Readers can order a copy by writing: Dr. Donohue – No. 1105, Box 536475, Orlando, FL 32853-4675. Enclose a check or money order (no cash) for $4.50 U.S./$6.50 Can. with the recipient’s printed name and address. Please allow four weeks for delivery.

DEAR DR. DONOHUE: I need help. I am an assistant principal in a large middle school. An irate bus driver called me to her bus full of students, furious with two boys sitting in the front seat. She explained that the two pass gas on purpose every day. Is it possible to pass gas on purpose? I want to educate them about this. – S.P.

ANSWER: It’s impossible to willfully generate gas. It is possible, most of the time, to control its expulsion until a person is in a more appropriate facility, like a restroom. Explaining that to the boys should clear the air.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475.