Red blood cells have a long life -- up to 120 days -- so rebuilding someone's red blood count is a long, slow process. The speed with which bone marrow makes new red blood cells is also affected by factors such as the type of cancer, type of treatment -- particularly the type and dosage of chemo -- and the patient's general state of health.

One thing to keep in mind is that red blood counts won't drop immediately after chemo but will start dropping after a week or two, and will then continue to fall for several more weeks. This is because chemo doesn't kill off the red blood cells already in the bloodstream, which are mature and aren't dividing rapidly. It kills off the cells forming in the marrow, and therefore there's a delayed response that corresponds with the rate at which the marrow is creating new cells. The rate of regeneration can be affected by the patient's age and overall health. Also, some types of cancer and some types of treatment (such as radiation and some medications) suppress the production of red blood cells, so the rebuilding process can be much slower.