People being treated for cancer often become anemic, meaning their blood oxygen levels fall too low, which is partly why they feel exhausted during treatment. To ease the anemia — and the fatigue — doctors prescribe erythropoietins, which stimulate the production of red blood cells.

On Friday, the Food and Drug Administration warned that these drugs, sold under the brand names Epogen, Procrit and Aranesp, are doing more harm than good in some cases. Doctors have apparently been over-prescribing the drugs, using them to reverse anemia, instead of just alleviating it enough to avoid blood transfusions.

The agency says that the drugs carry a higher risk of blood clots in the legs and the lungs, could make tumors grow faster, and could even cause people being treated for cancer to die more quickly. The drugs will now be sold with a black-box warning that highlights these risks.

Here’s what I don’t understand: we first reported on the risks with these drugs in 2003, when a couple of trials unexpectedly showed that people taking erythropoietin died faster than those taking the placebo.

We quoted experts who had found that many tumor cells have receptors for erythropoietin and that the cells grow faster in response to erythropoietin, and we reported — perhaps naively, in retrospect — that the mounting evidence might have an effect on how the drugs are prescribed.

As I said, that was in December 2003, more than three years ago.

Why has it taken so long for the FDA to act?