DEAR DR. ROACH: I am a 55-year-old male. After five or six days of thinking I had pulled a muscle in my bicep, I was diagnosed via ultrasound with an unprovoked, extensive deep-vein thrombosis in my left upper arm. I’m glad I made the decision to have it checked out. I have no family history of this condition, exercise regularly, am not overweight and have not had any recent surgeries. During my hospital stay, I had a CAT scan of my abdomen and pelvis, CT of my chest and an ultrasound of my lower extremities. These tests results were all normal. In addition, many blood tests were run, and there is no indication of any cancer.

I have been taking Xarelto since I was released from the hospital. The hematologist/oncologist indicated that we might never know what caused the DVT. He said that there is an increased possibility of another clot developing in the future, so I may need to take Xarelto indefinitely.

I would like to know what caused the clot. Do you have any advice/suggestions as to how to proceed? Does Xarelto specifically address this condition? — B.R.

ANSWER: A deep-vein thrombosis is a blood clot. Blood clots in the upper extremities are much less common than in the leg and pelvis. I’m going to exclude blood clots related to medical procedures and devices in my discussion, since these are very different in cause, treatment and prognosis.

Primary (where no secondary cause is identified) upper-extremity DVTs often have an underlying cause. Probably the most common identifiable cause is thoracic outlet obstruction, a compression of the nerves, arteries and especially veins as they go through the thoracic outlet — an anatomical space bordered by the spine, the first ribs and the breastbone. Young men with large muscles are at highest risk for blood clots in the subclavian veins that pass through the thoracic outlet. Sometimes, numbness and weakness are present, showing the nerve to be affected by thoracic outlet obstruction. If there is evidence of compression (usually by X-ray), surgical treatment should be considered.

For people without thoracic outlet obstruction, an identifiable risk factor can be found in up to 60 percent of cases (you probably had a search for these through the many blood tests you had), and as many as 25 percent will have a cancer identified in the year after diagnosis (which is why your doctor looked so carefully for cancer).

Most experts recommend anticoagulation for three to six months, and 2 to 8 percent of people will recur. Xarelto (rivaroxaban) is one choice, but is not necessarily better than other options. I’m not sure why your hematologist is recommending indefinite therapy.

DEAR DR. ROACH: My doctor states that my body could tolerate 3,000 milligrams per day of Tylenol. I am 87 years old and in poor health. Is he correct? — R.B.

ANSWER: Depending on the exact reason for your poor health, your doctor probably is right. A daily total limit of 3,000 mg of acetaminophen (Tylenol) is generally considered safe for most adults. However, in the presence of significant liver disease or in people with heavy alcohol use, a limit of 2,000 mg is safer.

TO READERS: Questions about the common problem of uterine fibroids are answered in the booklet of that name. To obtain a copy, write: Dr. Roach — No. 1106, Box 536475, Orlando, FL 32853-6475. Enclose a check or money order (no cash) for $4.75 U.S./$6 Can. with the recipient’s printed name and address. Please allow four weeks for delivery.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Health newsletters may be ordered from www.rbmamall.com.