DEAR DR. ROACH: My brother had four ministrokes in five days. They can’t find what caused it. He was in a university hospital for seven days. — M.L.

ANSWER: A stroke is defined as a neurologic deficit (such as weakness or difficulty speaking) that lasts for more than 24 hours. For deficits that last less than 24 hours, the term is “TIA,” for “transient ischemic attack.” Both TIAs and strokes are most commonly caused by poor blood flow to a specific area of the brain. This can be due to a blockage of the artery, leading to a blood clot (called a thrombotic stroke), which is analogous to a heart attack. A stroke also can be caused by bleeding from a damaged blood vessel (called hemorrhagic stroke). The other most common cause of stroke is called an embolic stroke, where small bits of solid material, such as clotted blood or cholesterol plaque, go to the brain and prevent blood flow to an area of the brain.

Multiple small strokes (we sometimes use the imprecise term “ministroke” for a stroke that isn’t disabling) suggest embolic strokes, although thrombotic strokes sometimes get better and worse, which can be confused with separate strokes. All of these types of strokes and TIAs need urgent evaluation to try to find the underlying cause and to prevent further events. An MRI is usually the first test done after a careful history and physical. When the cause isn’t identified, as in your brother’s case, further tests to find a source of emboli (the debris that blocks the arteries) are done, including a look inside the heart chambers and valves (with an echocardiogram) and at the wall of the aorta (often with a transesophageal echocardiogram).

Without knowing the cause, I can’t comment on the exact best way to reduce the risk of future strokes, but some advice is universal: Careful blood pressure control (including NOT lowering it too much too quickly) is essential, as is quitting tobacco in smokers. Control of cholesterol and diabetes through a good diet and medication, if necessary, also can help. Some people will benefit from aspirin, clopidogrel (Plavix) or related medications, or from statins, but only his doctor can answer what is best for him.

DEAR DR. ROACH: I’m in my late 60s and am concerned about the many antibiotics I’ve taken to treat my urinary tract infections. I’ve read that a supplement, D-mannose, helps to prevent these infections. Some sources suggest taking cranberry extract with D-mannose; others discourage this. I’d appreciate knowing more about this supplement, including the correct dosage amounts. I have a hard time swallowing capsules and would prefer to take the powder form of D-mannose. — Anon.

ANSWER: In a 2014 study of women with recurrent UTIs, one group took D-mannose, 2,000 mg, once daily in the evening mixed with 200 mL of water. A second group took an antibiotic, nitrofurantoin, to prevent UTI, and the third received nothing. The results showed 15 percent of the D-mannose group, 20 percent of the antibiotic group and 61 percent of the control group had a UTI in the six months of the trial. D-mannose is thought to work by preventing the bacteria from sticking to the wall of the bladder, which also is how the chemical in cranberry juice or extract works. Cranberry juice was shown to be superior to no treatment in some studies. However, a more rigorous study showed no benefit to cranberry treatment, and it is possible that D-mannose also will fail when put to more rigorous study.

Both D-mannose and cranberry extract have a low likelihood of side effects and are reasonable to try, but I would recommend starting with just one, not both.

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.