Anemia, or a shortage of red blood cells, is a common side effect of many serious diseases, including cancer. To treat anemia, doctors often prescribe drugs called erythropoiesis-stimulating agents, or ESAs. Now, a new study calls this practice into question, finding that ESAs create a higher risk of blood clots, which, in turn can cause stroke, heart attack and other problems.

ESAs increased the risk of death by 10 percent and the risk of a type of blood clot known as a venous thromboembolism (VTE) by 57 percent, according to the study data, which are reported in the Feb. 27 issue of the Journal of the American Medical Association.

Some doctors, however, defended ESAs, saying that they are safe when used properly.

"If you use ESAs the way they're supposed to be used, I really don't see clinically what they're talking about in the trials," said Dr. Jay Brooks, chairman of hematology/oncology at Ochsner Health System in Baton Rouge, La. "I still think ESAs are extraordinarily useful and safe medications when used in an efficacious manner. I would be treated with these agents if I had cancer," Brooks said.

The most common ESAs are erythropoietin (brand names: Epogen, Procrit) and darbepoetin (Aranesp). These drugs work by causing the bone marrow to increase production of new red blood cells.

Concerns have been raised about these drugs in the past. For people on kidney dialysis, studies have shown that if ESAs are used to raise hemoglobin levels above a certain level, the risk of death increases. Other cancer research suggests that ESAs may stimulate the growth of cancerous tumors.

Last year, the U.S. Food and Drug Administration added a "black box" warning, indicating that ESAs should be used at the lowest possible doses to avoid risks such as blood clots, heart attacks, stroke, congestive heart failure, increased tumor growth and an increased risk of death.

The new study, conducted by a team led by Dr. Charles Bennett, the A.C. Beuhler professor of geriatric medicine at Northwestern University's Feinberg School of Medicine, reviewed 51 phase 3 clinical trials completed between 1985 and 2005. Survival was evaluated in 13,613 people with cancer, and the risk of VTE was evaluated in 8,172 people with cancer. The type of cancer varied.

Overall, Bennett and his colleagues found the risk of VTE increased 57 percent in people taking ESAs, and the risk of mortality increased 10 percent.

"At the end of the day, these data are very provocative and it's important for people that make clinical guidelines to review the data," said Bennett.

"Patients should be informed of the risks and benefits of these drugs," he added.