Risks vs. Benefits. Hydroxyurea is used to treat sickle cell anemia and some leukemias (blood cancers). As will most medications, there is no official time limit. A medicine that is helpful, is doing a good job, has no equal alternative, and thus is needed, can be used until it is no longer needed. The doctor can explain the risks of "not treating" and of drug side effects, versus the benefits of using hydroxyurea.