Recent research has identifed several major hazards of ESAs:

Cancer progression
ESAs can make cancers of the breast, cervix, lung, head, neck, and lymph nodes progress more rapidly, according to several trials of people undergoing chemotherapy for those malignancies.

Cardiovascular problems
ESAs can raise your blood pressure by constricting blood vessels; and cause clots by thickening the blood. For example, a 2008 analysis that combined the results of 51 trials involving over 13,000 cancer patients found that 7.5 percent of those taking an ESA suffered a blood clot, compared with 4.9 percent of those not taking one of the drugs. That's a 2.6 percent increase in absolute risk, and a 57 percent increase in relative risk. Other research has found similar risks among people with kidney disease treated with large doses of the drugs in an attempt to bring their hemoglobin levels to normal (more than 13 g/dL), as well as in those who took an ESA before undergoing orthopedic surgery. Those effects help explain why the drugs increase the risk of heart attack, stroke, and heart failure.

Higher mortality
Two other comprehensive reviews, which combined the results of 53 prior randomized trials including a total of nearly 14,000 patients, found that people with cancer-related anemia who took ESAs were 10 to 17 percent more likely to die during or within a month after the study than were those who did not take the drugs. For example, the largest of two clinical trials identified mortality risks of 61% and 32% among cancer patients receiving chemotherapy or radiotherapy compared with mortality rates of 52% and 24% among cancer patients not taking ESAs, respectively. That's an 8% to 9% increase in absolute risk and a 38% increase in relative risk.

In chronic kidney-disease patients, a recent study showed a 17 percent increase in relative risk and a 4 percent increase in absolute risk of all-cause mortality among those taking ESAs to achieve higher hemoglobin levels (about 13 g/dL) compared to those with lower rates (about 11 g/dL).

In July 2008 the Food and Drug Administration clamped down on ESAs, particularly in treating anemia in patients with cancer. Using expanded regulatory power, recently granted by Congress, the FDA for the first time dictated to manufacturers the exact wording of beefed-up warnings in the drug labeling. Under the heading "What is the most important information I should know?" the medication guides for patients bluntly state that the drug can "lead to death or other serious side effects." Even more important, the revised labeling imposed major limitations on the use of these drugs for patients with cancer.