Robin Miller, a 62-year-old oncologist in Atlanta with bladder cancer, was scheduled to receive a potentially lifesaving drug in December. But her doctor’s office called shortly before the appointment to say: “Sorry, we don’t have any. We can’t give it to you,” according to Dr. Miller.

The disruption was due to a global shortage of the drug, BCG, which arose after manufacturing problems at two of the few global suppliers. Without the drug, Dr. Miller feared her cancer would come back and she would have to have her bladder removed, a step she called “barbaric.”

The BCG shortage followed a 2012 mold infestation that halted production for more than two years at an aging factory in Toronto owned by France’s Sanofi SA . The drug’s only other manufacturer for the U.S., Merck & Co., has recently suffered production delays of its own.

Some of the thousands of patients who depend on BCG have resorted to tracking online message boards, calling hospitals and traveling hundreds of miles to find supplies. Some patients have gone without BCG while others have received less effective alternatives. “There are patients who aren’t getting optimal therapy right now,” says Dr. Edward M. Messing, a urologist at the University of Rochester Medical Center in Rochester, N.Y.

The crisis illustrates the potentially grave consequences of a persistent problem in health care: drug shortages. The number of drugs in short supply in the U.S. has risen 74% from five years ago, to about 265, according to the University of Utah’s Drug Information Service, which tracks supplies. They range from antibiotics and cancer treatments to commodity items such as saline.

Interviews with company executives, hospital pharmacists and regulators point to several causes of the shortages. Companies have failed to build enough production capacity, haven’t maintained equipment, and failed to ward off contamination in aging plants. A U.S. Food and Drug Administration crackdown on shoddy quality unintentionally worsened the shortages because some companies responded by shutting down plants or scaling back production during renovations.

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