A shortage of a key leukemia drug that started last year has worsened, causing many major cancer centers such as the Johns Hopkins Hospital to start rationing the drug and others to turn away patients from community hospitals that have run out of the medication. The three companies that make the drug, called cytarabine, have all suffered production difficulties in the past year. Only one of them, Hospira Inc., is currently shipping the drug, but only in limited quantities that are not nearly enough to meet demand. A shortage in 2010 of the active ingredient used to make cytarabine slowed production at Hospira. "I really think what's happening is a national tragedy," said Hagop Kantarjian, MD, chairman of the leukemia department at the University of Texas M.D. Anderson Cancer Center in Houston, one of the world's leading cancer-treatment facilities.